<p>I have a semi-diy hotend setup on my MOOZ-2 (an obscure chinese printer). It requires that the total nozzle length (including threading) is longer than 15 mm. So far, I've been using Volcano nozzles because they're the only ones I can find that meet this requirement. However I believe this negatively impacts performance because they stick out way beyond the heater block (see picture below). Unfortunately, the next size I can find are MK8 or V6 nozzles which are 13 mm - too short! Are there any nozzle sizes in between that I could use?</p>
<p><a href="https://i.stack.imgur.com/IN5df.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/IN5df.jpg" alt="Nozzle setup" /></a></p>

I think this is just an overcomplicated lost-PLA (investment) casting.
What you're asking for is to create an object, create a mold around it, and then burn out the object and replace it with metal.  Traditionally this is done with wax,
and called lost-wax casting
, but the same can be done with anything that melts/burns away, including PLA.
Rather than worrying about burning hairs and pressure and compaction of metal powder, print a model, and use the correct kind of plaster (a search for "investment casting plaster" will get you going down the right path) to make your mold.  Heat the metal powder in a crucible, instead of the mold itself, and pour it through the expansion/extra material tube you were talking about.