If i have a speaker cab that can handle the volume, will it work to put a boost pedal between the speaker out and the cab's input so as to add volume with out changing tone, kind of like the opposite of an attenuator????? Has anyone done this? seems like a great idea in theory...
no, its a stupid idea in theory too.

firstly: the boost pedal would lead to an ohm mismatch. kaboom

secondly: the output of the valve junior would be too large a signal for the pedal to be able to handle. you'd fry the hell out of it.

thirdly: the output of the pedal would be too weak to drive a speaker. fail.

fourthly: Once the pedal fries/catches fire/melts, you'll have nothing going to the speaker because the dead pedal with break the circuit, and because there will be no load on the amp, it will probably blow out all the tubes at best - at worst, it will make the amp catch fire too.

Don't do it.
I like analogue Solid State amps that make no effort to be "tube-like", and I'm proud of it...

...A little too proud, to be honest.
That was kind of harsh, but he's right.
A regular boost pedal is meant to boost a guitar-level signal, not the level that comes out of your amp. That's why you need speaker cables and not instrument cables, there's a lot more power going through them.