tyres

Hitchcocks (the Enfield people) are showing TT100 350(410)x19 in stock. Best be quick if you want it badly!
Going forward it doesn't look too promising for the 19" users.

Mitas H-02 400x19 is on 'vintagetyres' at £81-60.  Firestone at £224!  But as you say - nothing at 410-19.

Whilst I agree 4.10 is seemingly almost impossible to get, you need to remember that it is really a 3.5" tyre - the 4.10 came about as the K81 was the first 'low profile' motorcycle tyre available with an 85% aspect ratio.
So, it can be worth searching for 3.50x19, or for the metric 100/90 or 110/90 widths.
The single biggest issue for Commandos is the load rating and speed rating of what is available at the moment.
The later Commandos aren't lightweight and the load rating needs to be in the 60's and probably a speed rating of at least an H to be 'safe'.

I too have been looking for a new rear tyre without success.  In the course of my research I have been told, twice, that it is acceptable to use a front tyre on the rear provided the direction of rotation has been reversed.  Can this be true?  I can't imagine manufacturers would specify front or rear if it was.  Can anybody enlighten me?

Looking without success?
4.10 X 19 TT100 We have in stock and there are no issues getting these from distributors.
100/90 - 19 Avon Roadrider MK2 we also have plenty in stock at the moment. 

Try telling your insurance company or accident investigator that it is acceptable to use a dedicated front tyre on the rear by reversing the direction on the tyre. If that was the case tyre manufacturers would highlight it. It depends on how the tyre is laid up and the technology they use to manufacture the tyre, that is what makes a tyre universal or not.