Most large scafold distributers have sales engineers who can help. I use Waco Scafold for a lot of unique shoring applications. You can find them at http://www.wacoscaf.com/
If the website does not have what you are looking for call them and they can help. Even if the contractor has pipe scafolding, it is important that it be rated for a maximum allowable load, Access scafold is different from shoring scaffold and should not be used for shoring. Finally a scafold system is only as good as its base
Good Luck

contact the joist manufacturer.  The SJI has details for bottom chord bearing joists.  Work backwards from the capacity of the joist for the given span and maximum loadings.  Calculate the approximate moment of inertia using the maximum live load for the given joist.  Work backwards from that to calculate the approximate sizes of the angle iron used in the joist flanges - perhaps you have already measured them in the field?  (often times the top chords are different from the bottom chords) based on your calculations find the maximum negative bending moment that the joist can handle at the point you want to temporarily support it - you will probably need to add a brace from the point of shoring at the bottom flange to a panel point at the top flange.  Again see the detail for bottom bearing joists in the SJI AND talk to the joist manufacturer to troubleshoot your approach.

Thanks for the information. I used timber blocking bearing directly on the top chord angles, and included working loads requirements for the staging and its components.

Ron