Singular Cycles

Yup, I do, and I have not had any issues since I built it ~2 years ago and I've ridden it through two filthy winters, but I was sure to liberally fill the last ferrule with waterproof (proper marine stuff) grease before fitting as it was obvious that something there water was going to drain down into that loop under the BB.

Probably helps that I run full guards & flaps though so I'm not getting as much crap sprayed at the cable compared to you. Even if you're not going to run guards t might be worthwhile making a mini shield to protect that cable stop area, you could just use a bit of plastic bottle or some other flexible plastic and bolt on the chainstay bridge, even on the inside, and then ziptie/stick it to the seat tube. It wouldn't be perfect but anything to reduce the spray going at that cable exit would help.

Other options are to run one of those ferrules with a built in extension and then slip another rubber seal over the end, or run an unlined outer and run a full length of liner all the way up to the front derailleur clamp.

Might a couple of the little rubber sheathes that cover the cable between the arms on a v brake help keep the slop off? I've seen someone on here solve a similar problem with cable disc brakes using that solution but maybe there's too much exposed cable here.