While hiking up Mt Ranier I kept thinking about how I should redesign my metal extruder, and eventually near Camp Muir I arrived at a solution.
Printing metal filament is nothing like printing plastic filament: plastic is a viscous liquid which forms its own sealing surface as it passes through its glass transition temperature. Metal may have semisolid states where there are solid and liquid phases are mixed together, but it is possible to separate these phases, causing granular jamming and clogging nozzles. This can be avoided by printing above the liquidus, having material only pass into the semisolid state as it goes over a deposition brush.
If one attempts to print metal in a fully liquid state, the thin liquid will leak out of any hole provided: the metal does not go through a glass transition, and therefore it does not form a sealing gasket surface. It is possible to have extremely tight tolerances on the metal filament and on the extruder, but then any variation in your filament would induce a clog in the nozzle. The required precision would make metal filament far too expensive for the average maker. Rubber gasketing could in theory take the place of the self-forming sealing surface on the metal, but it would be limited to low temperatures and the trapped air would tend to expand when heated, causing the nozzle to drip.
This is how I arrived at using a liquid glass gasket: design low melting-point glasses which surround the metal wire as it enters the hotend and before it itself melts, to restore the gasketing behavior that thermoplastic normally has. Serendipitously, I discovered a few such compositions myself long ago when playing around with a hotplate, and a quick look through google scholar shows plenty of options for these low-melting glasses even below 400C. The vast majority of these glasses are less dense than the metal alloys I plan to work with, except maybe in the case of aluminum, where care must be taken to pick the proper glass. This is important because the liquid glass gasket remains in place due to buoyancy on the molten metal in the nozzle and a few other factors, and the molten metal remains in place because of the capillary forces in the brush, as I learned all too well when playing around with the MHD system. I'm getting some key parts to test this concept with machined at the moment, but it won't be long before this method gets tested.