What we will want from Bitcoin Core (and other wallet libraries) is the ability to restrict coin selection to, at max, some total input weight.
We are basically 90% there. Our coin selection algorithms already take a max_weight parameter for us to be able to avoid exceeding the standardness limit for weight. Passing something through the transaction building RPCs should be fairly easy. CoinGrinder would be a big help though, since SRD only greedily searches for a solution below the weight limit by removing the lowest effective value input when it is overweight. BnB and CG should always find a solution if it exists.
Of course, CoinGrinder has been open since July and has seen only a modicum of review so far, so a few more people would have to take a look at it.