For the record, while putting modprobe fbcon at the beginning of /sbin/preinit does make framebuffer console load even when plugging in a turned off N900 to charge, I find that to be just fine. vi_'s post a couple posts above mine, though, is probably the right way to do it if you want a recovery shell only during 'normal' boots.

- Edit -
By just fine, I mean to say that even with kernel-power, it loads\oots/charges correctly in all situations I've dealt with (I might be wrong on that, of course, but I don't notice any issues.