there's nothing preventing this from working, except that right now, if there are fabric links (as opposed to mesh links) we make leaves only advertise their own VTEP, as opposed to all the VTEPs they know about. Which means that if you attach a gateway to the leaf, other leaves will not know how to reach it.

we could always advertise all VTEPs (we will do some extra advertising that would be useless because the spines would always win due to shorter AS-PATH) or we could add the VTEP of the attached gateway to the prefix list of allowed VTEPs for that leaf, which is probably a saner solution, if somewhat more convoluted