You can test anything anywhere if the leads are long enough. Sorry I'm being facetious, I doubt you would be able to do an assessment without dedicated kit & as I've said elsewhere we are blessed with Paul at Technozen when it comes to MG Rover electronics. I would contact Paul, explain your predicament & gauge his response.
I can't recall if your car is a VVC, if it is & has the coil pack mounted to the rear of the engine, I would change that & the plug leads. If you find yourself in North East Lancs at any time PM me & you can have coils, leads, airbag controller & any other bits in my bit box that might take your fancy. I'm not sure how much your catalytic converter would appreciate a gob full of shit, it might burn out whatever its ingested but that might need changing.
M
Oh & your idea regarding dehydrator plugs goes to the top of the ideas chart !