Anyways.. after thinking I made this:

The idea being that the threaded ring gets screwed into the case and this is aluminium. I am going to bond it in with Loctite or similar or as time goes by am I sure it will get stuck like everything else screwed into the case.
Will look like this when/if I do it:

Popped a nitrile 'O' ring in it for sealing.. also made it the right size to sit into the counterbore in the case.
Then I should be able to screw the new cap into it and tighten with a 13mm socket so it looks like this:

This cap also sits in the biggest counterbore of the case and looks flush when tightened and the 'o' ring compressed.. this was only finger tight.
So... the thinking is that I can still get a 17mm socket through the middle for timing etc, a 13mm socket to tighten and untighten the bung.. and as they are the same material and nicely anodised there should be no oxidisation/galling etc and the problem of the bung getting stuck in the case through mismatch of materials ( that other people have stated is the cause) should hopefully be there no more? The 13mm hex I was hoping should also be more of a positive drive.
The internal thread would be M26x1.5 and I have seen other sump plugs etc in this size and other blanking caps from other applications so should be able to find alternatives for the bung a bit easier.
Here is a cross section of the bung and I am sure it should be strong enough with these wall thicknesses.

also for Tony's sake I have a picture here as I know he likes to know these things... this is the whole assembly.

I'd welcome any thoughts good or bad so I can maybe refine it etc before anodising and bonding it into the bike...