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.

I make a nut free frangipane using rice flour in place of almond meal. I think a mix of this with egg, sugar and butter would work. The rice flour has the same slightly gritty texture and doesn't add gluten to the mix.

You could probably try one of a number of different nut-free flours mixed with the almond flavoring (likely water, ethanol, and benzaldehyde from a chemistry standpoint). Thinking about what I have in my kitchen, maybe rice flour? Cornstarch, tapioca flour, and potato flour would be too fine in texture.

You could try to make a roux and brown it for a nutty flavor. The texture might be about the same too.

Add sugar, you don't want it to turn to caramel, so try to add on low heat or no heat. That will account for the sweetness of the nuts.

I'd add the nut-free almond flavor once it cools.