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I agree this is a bit surprising.  When I was writing this
code I went for the conservative-as-possible approach as I
didn't want to introduce instabilities to Python.

It certainly may be that I've overdone it.  In this case I
probably have; if the tp_dealloc of the class being adjusted
is subtype_dealloc and the tp_dealloc that ultimately gets
invoked is the same we're probably OK.  But I'm not sure and
it's been a while since I thought about this.

It also happens that my motivating use-case for this isn't
troubled by this restriction.

I don't understand your last, parenthetical, comment. 
HEAPTYPES as such doesn't come into it, does it?  

You might be right that we don't need to worry about
tp_dealloc if the ultimate solid_base doesn't change and all
the tp_deallocs on the way there are subtype_dealloc...