I agree with theBFG. I think it should be something that is offered and not asked, but then I've never been in the situation where I've needed to ask such a big 'favour' and I suspect not many would be discussing such a personal issue like needing a surrogate openly enough for a close friend to offer IYSWIM.
On the otherhand, if it were a very close family member that asked me, brother or sister for instance, then if I were able then I probably would consider it.
Such a confusing and emotive issue.