Is MCrypt's 8-bit OFB mode secure?
Actually, I think I found the answer to my question while writing it, but I'll post it anyway, since it might be interesting to others:

Yes, OFB mode is secure even with 8-bit feedback, at least as long as IVs are chosen randomly.

Specifically, in the paper "New proof for old modes" (IACR Cryptology ePrint Archive, 2008), which I've cited earlier here, Mark Wooding proves the IND-CPA security (and, in fact, a somewhat stronger security property termed ROG-CPA, for "real or garbage" indistinguishability) of both CFB-$n$ and OFB-$n$ modes for any number of feedback bits $n$, provided that either:


the IVs are chosen randomly,
the IVs are generated by encrypting a deterministic sequence (a "generalized counter") using the block cipher, or
the IVs are directly chosen according to a deterministic sequence, and $n$ is no less than the block size of the block cipher used.


(The extra limitation on the feedback size $n$ in the case of a deterministic IV sequence is due to that fact that CFB and OFB modes using less than one cipher block of feedback have some rare "weak IVs" that can produce a low-period keystream.  Presumably, a deterministic IV sequence chosen to deliberately avoid such IVs would also be safe.)