Alright, I just meant "Not now"...would Ní anois be grammatical there as well?

Ní anois; ní ag an tráth seo; ní ag an bpointe seo...are a few ways of saying 'not now'. I think it would depend on the context.

Ní dhéanfaidh sé é. Ní (díreach) anois.
He won't do it. Not (just) now.

Ní dhéanfaidh sé anois é.
He won't do it now.

Ní dhéanfaidh sé é ag qn tráth seo.
He won't do it now.

Yes. You can leave anything out following a focussed phrase as ní anois,
Ní anois (atá mé á dhéanamh).
So, ní anois on its own is grammatical.
But you must be aware: It's an ellipsis.

English "not now" isn't an ellipsis (or rather a different kind of).
In English, "not" is an adverb. It can negate any sort of words (not I, not him, not that, ...)

But Irish "ní" is a verbal particle or a copula form.
As a verbal particle, it can only negate finite verbs (ní dhéanann sé, ni thuigim, ní fhuil tú > níl tú)
As a copula form, it can only negate copula sentences.

Ceart go leor, a chairde, go raibh maith agaibh. Agus Nollaig Shona dhaoibh!

One final question: are there much more indirect ways of saying it (perhaps sula dtarlaíonn sé nach...), and if not, is ar eagla nach the best translation?

Anyone?

Really? Nothing?

sarar dtarlaíonn sé = lest it happens.
I had a look in Dinneen Dictionary and sarar is given as an alternative for sular which it gives for lest. you may download a free version of Dinneen' s dictionary and it is excellent .McKenna English-Irish Dictionary also gives SAR for lest

OK, thanks.