One could do what Ohio would occasionally do in the past and mount a single sign on the side but on a cantilever over nothing.
https://goo.gl/maps/yZq9RzLLR3XSn4Dj8Mass replaced full gantries with cantilevers for 2 full-size BGSs in places on the Mass Pike also.
https://goo.gl/maps/75uCHBBwFCUu1FdU6 (It is interesting that the median mount for the old gantry wasn't removed fully; the uprights must not have wanted to un-bolt or were frozen in place, because they just cut them off and left the stubs.)
They did the cantilever eastbound as well (
https://goo.gl/maps/ghh6XmY2Zir9wR7W6) and street view came along at just the right time to catch the old signs, still on the cross member, sitting on the side of the road. (It can be seen from the EB gantry post-sign-removal that they just cut the vertical members here too. Easier.
https://goo.gl/maps/kF7qVcSJSgBf4Aot9)
It seems like they only used to cut off gantry poles when they were totally immovable, like this remnant that has been in the median of 128--I mean I-95, for over 2 decades now.
https://goo.gl/maps/crjLMU5uf285LGdu9 The larger metal circles were the two parallel vertical parts, and the smaller one was a diagonal one as there were diagonals zig-zagging up between the two verticals. That gantry was removed and replaced with a full-dual-carriageway-width one which when new had button copy numerals for 93 (so we're talking a while ago); the full-roadway gantry and signs have been updated again but that bugaboo of the 70s gantry having a few feet concrete poured around its base has left the cut-off poles there until whenever they redo that median wall (which probably will happen eventually, but not for a while knowing Mass).