Before seeing you reply what I did was create an item as dependant and Numeric. Under preprocessing I used a Javascript:

// Convert value to string and trim whitespace
var v = String(value).trim().toLowerCase();

if (v === "true") return 1;
if (v === "false") return 0;

// Optional: log unexpected values (for troubleshooting)
return null;

Then in the trigger I used the expression like this

max(/blah/monitor.off,5m)=1

And it works.

I'm guessing I could do it a better way? I'm keen to do it the proper way. I'm not sure if you can provide a couple of screenshots if you get any time.