The mold fill problem is not due to the zero freezing range.  For example, a eutectic composition will have a zero freezing range, and fills the mold all the time. Cast Iron is an example of this.

Mold filling may be due to the lack of superheating.  If your alloy is at 1 degree above the melting point, then it will cool down and solidify shortly after it leaves the crucible.  

Also, there are other heat transfer considerations here.

Steel is a peritectic and it is cast all the time without mold filling problems.  Just another example.  

Hope this was helpful!

A peritectic reaction occurs when a liquid-plus-solid phase reacts to form a different solid phase on cooling.  Thus in casting a peritectic alloy there will not be a zero freezing range.  The copper-zinc system is a series of peritectic reactions and litle problems are seen in the casting of the brasses.

The fluidity and castability of a eutectic alloy goes up at the eutectic composition, e.g. in electronic lead-tin solder.

A major problem in casting a peritectic is that the second solid phase will nucleate and grow on the first solid phase and shut off the diffusion necessay to maintain equilibrium.  This not a problem in casting steel because of the high diffusivity of carbon at the temperature where the liquid and delta-iron react to form austenite.

Your problem might br solved by preheating the mold and/or going to a higher degree of super-heat before pouring.

If you are pouring the peritectic alloy into a metal mold please ensure that there are enough vents for the air to escape and the mold to get filled. Alternately sand molds are reasonably permeable and with normal venting practice mold filling should not be a problem. The only other area you will have to look into is to you pouring practice i.e tapping temperature,ladle temperature time to pour etc. With this in mind it shall not be to fill a mold successfully.
Good Pouring