pro would be the neatest construction.
cf. e.g. Cato.Hist.83 tribuno militum parua laus pro factis relicta;
Liv.25.2.8 ludi Romani pro temporis illius copiis magnifice facti;
Pl.St.690 hoc conuiuiumst pro opibus nostris satis commodule, etc.
It is the disproportion between an action and the agent here that is important.
I’m not sure that pro puella works well in that sentence. It’s too ambiguous (there are how many hundred definitions of the preposition pro?) and it doesn’t make clear that Atalanta is the puella to whom reference is being made. Those examples fall under the basic meaning of pro “in proportion to”. In proportion to a girl? What girl?
You’re right on to try ut in my opinion. It’s quite a classy particle.
Atalanta, ut puella, celeriter currit.
Here it can mean something along the lines of “considering that” or “granted that”. It can even approach “although”.
Res publica, ut in tanto discrimine, non eversa est.
Sometimes those poor Romans use it for their μὲν…δέ = ut…non.
Now for the best part. If you wanted to add a verb, you’d get to use the subjunctive:
Atalanta, ut puella sit, celeriter currit.
“Atalanta–let’s grant that she’s a girl–runs pretty quickly.”
Which happens to be identifical to a purpose/result clause. Whoops!