You've hit the nail on the head with the structural ridge beam. All jabbing about prefab trusses aside, no one will do something atypical for you on the cheap, because there are just too many unknowns. I don't blame your engineer in the least for insisting upon enough of a fee to be able to properly tackle the situation. With this in mind, let's see if we can't simplify the situation...
I absolutely would not rely on a 2x10 collar tie in a situation like this: it would work, but for how long? One of the greatest strengths we have when we work with wood is how well it takes momentary overloads (wind gusts, seismic racking, etc), but the inverse is just as true. You would be flexing this joint continually for a number of years, and I would think it would eventually fail, at a minimum in the deflection serviceability limit state through rotation at the joint. Basically, you'd get a crazy bouncy floor, or perhaps even a dangerous structure.
You're certainly right in saying your design is atypical (always a Bad Thing, capital B, capital T), but you can easily improve it by bringing in as much typical framing as possible. Balloon framing is not the enemy it is made out to be: 18th & 19th century balloon framing is, but that involved notching members to provide bearing area. Don't notch your members and you fix the problem. Instead of notching, bring jack studs up both sides of your continuous member, and support your floor system on the jacks. Make the continuous member a deeper section, and calculate it's bending etc based on pin connections at base and at 2nd storey floor. You'll find that there is still more thrust than you think and that your connection between the column and 2nd storey framing is going to be a critical design concern. And make sure you do use a ridge beam (be careful in your detailing to ensure that it is a true ridge beam, not a ridge board: Load path, load path, load path.... Properly sized column to a properly founded foundation). That will remove a good deal of load from your roof, and will acheive a significant reduction in thrust. Although I'm still betting on fair size thrust under unballanced loading conditions. You're also going to have to be quite careful with your deflection limits, both on the ridge beam and on the vertical columns.
Take a look at getting diaphram action to carry your load from rafters which come down between columns accross to the columns at 4, 6 or perhaps 8 feet. I think you're going to have to keep the columns quite close in order to support the thrust in a reasonable manner, but 8 feet should be acheivable if you're willing to have a small bulkhead around the columns. Keep in mind that corners = cost when drywalling. Careful with diaphrams mixed with balloon framing: You need continuous top plate between area acting as a diaphram! Your engineer should be able to spec this quite quickly if he/she's familiar with wood diaphrams.
Collar ties in the traditional sence are hard to size and can be very problematic in the field. A tensile load transfer through a column to beam joint, with a ridge beam above to reduce overall tensile load as we've discussed may just be a realistic long term solution. Be careful to check your continuous third of your column in shear! Double it up if you need to, and DON'T allow services to pass through if they will be far from the point of inflection on your shear diagram. Wood does not like shear, and you won't like what shear does to wood.
Please don't take offence here, but I would encourage you to have this designed by a competent Structural Eng. in your area. Make the changes I've suggested (to simplify his/her life and save you money) and then get a stamp. You'll also find that all of your hard work could be for nothing if the local building authority refuses your plans without a seal...
EG: I think Dan's issue was that $500 still had not yielded a solution...
Dan: Good luck! Hope I haven't discouraged you too much, but I think you should get a SE to take a look for you. Please do not think that this is sufficient advise to procede on your own. I wish I could help you more, and assure you of what you should do; but this is just impossible without being there and seeing the plans. Also, given the abnormal construction, I would encourage you to find a SE who's willing to come take a look at the finished product. Do not rely on this advice for your design: Take advantage of it to re-work your design and make the SE's life easier. You've got an interesting idea, and I think it'll be a great place.... You do need advice you can rely on in order to acheive it. I really hope I've been helpful, and not too stressful!
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
