I think you will have to milk her a little, just to relieve the pressure. Also, you can get her used to a routine of being tied up, fed and milked, so that when the calf comes it's just one addition to an established routine
If you are happy to work with her to get her to accept the calf, I'd give it a go - but I know I can always get my Jerseys to accept a set-on eventually, even if it sometimes takes a couple of weeks, and I don't know your Dexter!
My process is to tie the cow up with some cake and set the calf on, doing whatever I need to keep the cow from kicking it off, three times a day if I can for the first couple of weeks. (If it's too traumatic at first, I milk a third feed off and bottlefeed that, just put the calf on the cow twice a day.) After about a week, I make sure the calf swings its bum under the cow's nose as it suckles, so she starts to smell its smelly bottom that now smells of her milk. If you scratch the calf under and around the tailhead, it will usually lift the tail up, so she gets more of a whiff.
If she isn't aggressive to the calf, I leave the calf in with her - they are very good at working out when they can nip in and get a suck
Best of luck