I'm going to give this a go on some Trial Tech SL rims. Had quick google and found a guy on the net who has managed it with these particular rims as I was initially questioning whether it would be possible with the rectangular stamped holes.
Interestingly this guy used some laminate flooring underlay foam to fill the dip in the centre of the rim and then used the gorilla tape over the top to build up the height to help with getting air in to the tyre initially - might be worth knowing if you're trying a conversion with more conventional trials orientated rims. Link here.
When I get round to it, process is likely to be:
existing rim tape, I just use a single wrap of aluminium foil tape - looks quite nice on the stamped rims.
potentially a layer of thin foam to build height - going to try avoid this if possible but looks like it might be necessary.
20" tube cut down the middle and stretched over the rim in order to make a tubular setup similar to Ali's, also take care of the valve.
DIY "huck-norris" made from a 15mm thick closed cell yoga matt (less than a tenner from eBay).
toss in some sealant.
cross fingers.
attempt to seat tyre with track pump.
swear, a lot.
accept failure and go back to tubes.