I like this idea!
One slight modification though - to make it more universally appealing, I'd do it with a humbucker sized pickup route. Those that prefer P-90s have a lot of great options for humbucker sized P-90s (HD, Fralin, Rio, Duncan, etc.). Options for P-90 sized humbuckers are far more limited.
And why not a sustainblock?
To me a Jr is all about one P-90 and a stop tail or a wrap tail, but a sustain block would give it more uh...sustain!