It’s appears technically impossible:
To transfer grin from inputs with total blinding factor i to a recipient output with blinding factor r, one needs to be able to sign the excess r - i, which requires knowledge of r. But anyone knowing r can spend the recipient output at will…
Addendum: technically, signing the excess requires only knowing r-i. So if one doesn’t know i, one can still sign the excess for transferring to an output i+k. But then one wouldn’t be able to construct a rangeproof for that output. So it’s the rangeproof that requires knowledge of the output’s blinding factor…