Ok ... in that case I understand the problem.
I have the same assembly on my bike.
Although its a 750 which had a steel swingarm as standard, I fitted an ally swingarm from an early 900 some years ago.
Some good news, I think .... you say the nut "is now jammed across (its) points in the channel/saddle washer".
I think this must mean that the shank of the bolt has turned slightly in the aluminium bridge pieces of the swingarm and so it follows that it isn't seized absolutely rock solid in there.
Of course it may well still be pretty tight because that's where the galvanic corrosion will have taken place between the steel bolt and the ally swingarm but it must have moved a tad so it can't be rock solid
... the solid seizing must be in the threads of the bolt and the nut.
This means that if you can remove the bolt head you should (hopefully) be able to knock out the remainder of the bolt using a suitable sized drift, as you have suggested.
I would be inclined to agree with your thinking to drill off the bolt head.
I had a closer look and a measure up.
My workshop manual says that the bolt is M10 x 1.5.
According to my tables, the root dia of those threads will be 8.16mm.
If you can drill at that dia (or preferably a tad larger) and slightly beyond any 10mm shoulder that the bolt will almost certainly have, then the bolt head should separate from the threaded portion.
Using a drill of that dia should avoid cutting into the ally, which is to be avoided at all costs
You'll need to keep the drill centralised and parallel for the same reason.
The drill should centre itself fairly well in the damaged hex socket head and the access hole in the swingarm will be a good guide to keeping the it parallel.
I measured that its 80mm from the outer face of the swingarm to the base of the bolt head, so allowing maybe 30mm of drill shank in the chuck plus a bit of clearance you'll need a drill bit at least 120mm long just to reach the base of the bolt head.
You might need a longer than std drill (the official term is "long series").
I would suggest that you aim to give it your very best shot first time.
If the drill is allowed to rub rather than cut it will only make the material tougher .. and its probably fairly tough already.
Beware of cheap drills .. I have seen some that were ground so poorly at the business end that they wouldn't cut butter. Dormer is a reliable brand and HSS (high speed steel) is the proper grade.
Don't run the drill at high speed as that will only increase its tendency to rub rather than cut. A nice (very) slow speed and a heavy feed pressure is best.
Some cutting oil would help too .. Machine mart used to sell it in litre bottles .. but somewhat counter intuitively, even engine oil is better than nothing. Maybe you know some local engineer who can give you a thimble-full. Or if you twist my arm I'll post you a small bottle.
Long series drills are expensive.
Here's an ebay listing where you'll find prices and a table of sizes / lengths.
www.ebay.co.uk/itm/153729537969
But I came across this guy who is selling a NOS Dormer HSS long series drill of 8.5mm dia for under a tenner. I reckon that's a good deal and possibly the perfect size as it should fit down the damaged 8mm hex socket without touching the sides (8mm allen key measures 9.1mm across the points so when your's spun it should have mangled the hole to about 8.5mm or more) giving you immediate purchase on the meat of the bolt as well as good centralisation of the drill.
www.ebay.co.uk/itm/126294754081
continued .......