I wanted to change windows to use the unabbreviated ante meridiem and post meridiem for A.M. and P.M., but they're one letter too long. Google Translate lists pro and ab as alternate translations for "before" and "after", respectively. Would using that make sense in this context? Thanks!
The Oxford Latin Dictionary lists syncopated forms for post meridiem: posmeridiem and pomeridiem (cf. Italian pomeriggio); both of these would be short enough for your needs. However, although OLD has entries for both, the only attestation I've managed to find so far is Quintilian, Institutio oratoria 9.4.39, which actually has pos meridiem in the version of the text (Winterbottom's 1970 edition) that appears on the PHI site:
Inde 'belligerare', 'pos meridiem' et illa Censori Catonis 'dicae' 'faciae'que, m littera in e mollita. Quae in ueteribus libris reperta mutare imperiti solent, et dum librariorum insectari uolunt inscientiam, suam confitentur.
(OLD cites this passage in the entry for meridies but prints pomeridiem instead of pos meridiem.)
As for an alternative to ante meridiem, OLD does list promeridie as an adverb, but the only attestation given is for an inscription (CIL 6.2104.20):
PROMERIDIE FRATRES ARVALES PRAETEXTAS ACCEPER<UNT>
Still, there's some support there for the prepositional phrase pro meridie that you asked about.
Otherwise, the only thing I can think of is mane ('early in the day', 'in the morning'), though I'm not sure how well it works here. I'd be interested to see other opinions.
Update: Overall, I think it makes the most sense just to omit the interword spaces, as Draconis has suggested in a comment. Not only did the Romans often leave out such spaces in writing, as noted in the comment, but antemeridiem as a single word has its own entry in OLD (but not in Lewis & Short), and the entry for meridies in OLD notes that post meridiem can likewise be a single word.
Pro means 'before' in the sense of 'in front of, in the presence of', as in he stood before the judge, but to my knowledge doesn't have a temporal sense.
Ab means 'after' in the sense of 'from, since', as in I've like snow since childhood, but that's all.
Neither quite has the meaning you're looking for.