> This makes my monitor sleep until the trisquel login manager (well, it > did when it could boot).
"until" means it works ok since showing the login manager? Try adding "nomodeset" to the kernel arguments, it will disable kernel modesetting and should show something different (e.g. a VGA terminal). I don't know if the 2.6.32 in Debian supports KMS for this GPU. Maybe try a newer kernel for Trisquel (e.g. from http://jxself.org/linux-libre/)?
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