All I can think of is move in with a step without extending your hands, that way you keep the guard and can react. This is a WC response, trying to achiever a bridge by closing down and getting to the side. If you let him control distance and angle you are finished.
The Tai Chi way is the best however, move in and stick to them, flow with their movements, jabs will not be a factor any more but you have to be ready to neutralise the hard hits that will come you way as you establish that bridge (My Tai Chi is still basic but I think stick and follow is a big aspect of the art). If you drop low for gods sake be like lightning as boxers will punish your head if you give it to them, once your past the danger point being in close to their body and low is grate for destroying their structure and putting them on the floor its getting their that's hard.
Personally I think the funniest thing you can do to a pure boxer is hit high and when he drops to bob under kick him in the gut as he wont see it coming lol No way that would work with someone who cross trains a lot but if they don't do kicks
When I used to train WC with Allen Gibson, we used to practice boxing combos on the pads for a few weeks and with partners, then we would be sent of to think up counters using the syle. Try them against your now reasonably proficient opponents. If it seemed unlikely to work for real, back to the drawing board. Test, Test and Test some more.
P.S. boxers are VERY good at what they do so don't play their game.
I would love to hear from more experienced people on this,