I think this shouldn't count as a humble form, but rather as a patronizing form - which is still quite respectful to the recipient, but it's only that usually implies that the speaker is in superior position than the recipient of this form.
There are several forms of patronizing requests, and I'm not entirely sure about the nuances of each and when exactly they can be used in non-patronizing contexts (as they do sometimes), but my impression is that all commands and requests that are based on 連用形 (renyōkei) forms with なさい or without anything at the end tend to have a patronizing element to them, even though it's not apparent in all contexts.
These forms include (the verb 行く will be used for example):
- お行きなさい！ (o + renyōkei + nasai)
- お行き! (o + renyōkei)
- 行きなさい！ (renyōkei + nasai)
The following are not patronizing requests:
As for お出で, this is a slightly more complicated case, since it doesn't really come from a verb. From the perspective of the modern language, you can say it comes from the verb 出でる, but you'll rarely find such a verb used alone (without the prefix お) and I don't think there was any such verb in Classical Japanese. Some dictionaries list the verb お出でる (including the prefix), but they explicitly say it's derived from the noun/adverb お出で.
My guess is that it comes from the Classical Japanese particle いで (which was used from invitation) or maybe from adding to the verb 居る (いる) the particle で (or even the renyōkei of the verb 出る which is also で). Either way, お出で was first formed as a keigo noun which means "being somewhere" or "coming or going". It then got used in compounds as if it was a renyōkei form of a verb (since renyōkei forms are themselves, in fact, nouns). Update: Read Tsuyoshi Ito's post - お出でる is probably derived from the classical verb 出づ.
I'd say that's the case here with お出で being used as a patronizing form. But I don't think any おいで coming alone would sound patronizing, and お出でください is obviously not patronizing but plainly respectful.