As I think about it (sorry, this is going to be a very England biased post)...
I fully support EngA being a bridge between age-grade and full international, rather than just 2nd XV.
I'd say that they should have a wider squad for the year, and join senior training for those 2-3 day camps through the season, overseen by Borthwick.
Limit selection to being U25 or so, but targeting players who best fit the gameplan - which should be part of building an England identity. So if (for example) England snr want a boshing 12 and a play making 15, then EngA should play a boshing 12 and a play making 15. If England play a kick-dominated game, and a blitz defence, then EngA should play a kick-dominated game, and a blitz defence etc.
We know that SA will brutalise you with their power game. Whether snr, A, age grade or women.
We know that NZ will play an all-court game. Whether snr, A, age grade or women.
We know that France will play a big pack, a petit-general at SH and jouer jouer. Whether snr, A, age grade or women.
For England, the U18 play a different game to the U20 - even when it's the exact same players 2 years later - who play differently to EngA, who play differently to snr