with the c system procedure (system call)

access (path, W_OK);

you could simply check if the filesystem in this path is writable (even the file), doing this for all files in the list before starting to remove (even one) file of the package:

for (lp = alpm_list_last(info->files; lp; lp = lp->prev) {
if (access(lp->data)) { // don't know if lp->data holds the full path...
return (1); // error case - file not writable, can't delete
}
}

for(lp = alpm_list_last(info->files); lp; lp = lp->prev) {
unlink_file(info, lp, targ, trans, filenum, &position);
}

A bit sad you have to go trough the list twice, but the only way in my eyes.

Kind regards,
Georg

if (access(lp->data, W_OK)) {

of course, sometimes a bit of a brainless fast-writer.

Added in CVS using mostly what Georg suggested above.

Tested by remounting /boot and trying to install kernel26. Pacman now prints an error message stating the mount is read-only.