Nut-free "Almond Paste" filling
I wouldnt't do a starch-based "paste" (it will be a pudding) because it will have a completely wrong texture, way too creamy. 

My first choice would be a bean paste. Make it with white beans instead of azuki to get a color more suggestive of marzipan, and add more sugar than usual to distract from the bean taste. A close second, if this is compatible with the allergy, would be chestnut paste. 

If these are not acceptable, you can also try a semolina pudding cooked in water, not milk. Some prebrowning as clurect suggests for his roux will help too. But this will not come very close.