I don't think there's any practical way to clean inside of frame prior to PC. Simply re-treat frame after PCing (oven baking may not actually hurt frame protection much, but it's simple to re-treat just to be sure).

Go to walmart, buy a lifetime supply of boiled linseed oil in paint dept. for $5 (1 qt). This is enough for 5 frames if you're wasteful, or more if neat.

After PCing frame, pour BLO into frame tubes, slosh around to fully coat all inner tubing walls. Plug holes with paper towel to aid in this process and reduce mess. Do DT,TT,ST, HT,stays.

Hang frame over old newspaper (ideally outside for smell), let excess BLO drain. Wipe it off external surfaces before it hardens, very difficult to remove once hardened, akin to sticker adhesive residue. Change orientation of frame often to get all the free BLO out of frame.

It will stink for about a week, at which point the BLO will have dried, and you can begin to build frame.

You can also use extra BLO as a cheap threadlocker (works good on spoke threads for wheelbuilding).