I've been giving a lot of through to this sort of situation recently.
I think there is a way forward and it sort of resurrects the 3/8 lo pro chain angle.
Before this was often looked at as a way of saving kerf and more efficient milling but it has been shown that kerf saving is minimal even from .404 to 3/8 lp chain.
But using a 3/8 lp chain does sap less power from the saw and it also seems to clear chip better than .325 chain. This is not an issue on a short bar but over length bars on smaller saws using 3/8 lp could be the way forward.
It would help keep chain speed up and would reduce the strain on the saw.
Logosol have used these for years and so it seems that certainly the Stihl 3/8 lp chain 63PMX can cope with the power (really 3/8 lp is designed for saws with less than 40-50cc).
Oregon now offer their 3/8 lp chain - and it looks really good with a short top plate, so ok the chain will not take so many sharpens but lots of space for chip clearance and smaller cutters give less weight and resistance.
I only have it on the custom chain finder at the mo here Custom chain loops
But no one makes these - you can get long bars with an .050 guage but no large 3/8 lp nose sprockets to go on the end of them.
Also there is the drive sprockets and rims - quite hard to find but am searching!
I'm speaking to Tom Beerens (One of the founders of GB bars who used to make the logosol 3/8 lp bars) into making some more of these style bars for just this scenario ie. you have a saw that is not quite large enough for the wood you want to mill, but you really don't want to buy another saw for £800-£1,000.
Lengths of bar would be 20", 28", 36" and 42".
How far off are these? I may be getting a few to trial in around 2 to 3 months (hopefully x3 of each for Stihl and Husky mount or possible a multi mount with spacers for Husky).
Cost? Not cheap but not expensive I would estimate:
20" bar would be £50-00+vat
28" bar would be £70-00+vat
36" bar would be £95-00+vat
42" bar would be £120-00+vat