I have something new to report after doing more testing.

It seems that if I launch weston with the pixman backend (weston --use-pixman),
that works, but meanwhile, startx with 'exec twm' in .xinitrc doesn't work on
first attempt, it causes the lockup.

However, after the lockup, if i try to startx again (still with twm), it
suddenly works and i can start applications in twm and use the desktop. I
managed to reproduce this with Xfce as well, the first 'startxfce4' after
bootup will fail and lockup the GPU, and after it resets, I try again and Xfce
works.

One thing of note is that when I finally do manage to get a DE started (after
the GPU has locked up and reset once), glxinfo shows only "gallium on
llvmpipe"; no hardware acceleration available.

So whatever is causing the lockup is something that X does at startup (even
when a minimal X window manager like twm is used), but weston-pixman doesn't
do.