Is 'ecce esse!' acceptable Latin for 'Lo, to be!'?
I've tried looking online for answers, but I've not found anything definitive either confirming or disconfirming that it is, though I do not have experience in this kind of research.
Yes, you can. In Latin you can always write ecce and a noun, as in ecce homo. The infinitive of a verb (such as esse) can be used as a noun. For oblique cases you would have to consider the gerund, but it is not needed here.
The phrase ecce esse will be a little difficult to parse, just as "lo, to be" is. It is a compact and unusual expression, but I see nothing ungrammatical about it.
Whether ecce esse is the best phrase for your purpose depends on context. As a translation of "lo, to be" without further context it is fine. In more specific situations there can be better ways to put it.