Solved! Go to Solution.
There isn't an official callback as such, but you can add one yourself if you want!
Something as simple as
Scope: Before Load Rules (run once)
with myCallbackFunction being defined within your own webcode
Or some variation of this anyway.
You can play around with the scope a bit too, ALR may work for you, as may DOM Ready (though may be a bit slow for your liking).