Anyway, what color/colors would you possibly get breeding a Buckskin Snowcap Appy filly (out of Buckskin Blanket mom and Bay Fewspot dad) to a homozygous black (Friesian) stallion?
Depends on what the buckskin's genetic makeup is. Doesn't matter if she's homozygous black, as the vast majority of Friesians are (and your question mentions this one is), so the foal will be black based. If the dam is homozygous agouti, foal won't be black, will be bay or buckskin. If heterozygous agouti, black and smokey black are also possible. Being a snowcap (assuming it's a true snowcap), the foal will be at least Lp, so will have appy characteristics. Might or might not get a pattern (likely spotted blanket if so). So: either 25% chance black, 25% smokey black (which usually just look black, maybe some subtle differences), 25% bay, 25% buckskin OR 50% bay 50% buckskin. Could be characteristic only, or could be patterned appy.
Or breeding same homozygous black (Friesian) stallion to a Bay Near Leopard filly (out of Bay Leopard dad and Chestnut Leopard mom)?
Like the above, depends on if dam is homozygous agouti (if so, only bays) or not (if so, black is also possible). (with a chestnut mom, this one is not homozygous black, but doesn't matter in this case). Being a near leopard: 50% chance of passing on Lp, meaning foal will have appy characteristics. If that Lp foal also gets the pattern genes, would have a patterned appy, such as a spotted blanket or near leopard. If it only got the pattern gene without Lp, solid foal. So: either 50% chance at black, 50% chance at bay, or 100% bay. 50% chance of solid, 50% chance of at least appy characteristics and possibly also an appy pattern.
I can't answer your curly questions, as I don't know the inheritance of curly. But I did just read a magazine article about curlies. One of the curlie registries doesn't allow outcrosses to be registered, but others do. So you may find some breeders won't allow outcrossing, but I would guess more do than don't. Just like the Friesians: FPS/FHANA doesn't allow crossing, but the other Friesian registries do. FPS approved stallions won't stand to non-Friesians, but others will.