Presumably you would remove the standing space from the cabins and load them from the corridor, a bit like deep shelving.
You‘d have to be a contortionist to do that in the current vehicles; the side corridor is rather narrow!
In the current vehicles, removing all but one of the Classic rooms in the regular bed coaches, and converting them to couchettes 4 to a room with one central ‘corridor’ to access the ’beds’ would add 2 beds to the current 20 per coach. Essentially 3 of the four classic rooms would go, replaced with 2x4 bed couchette rooms in the same footprint. That means a 10% increase in capacity in those coaches, of which there are usually 5 per half set on the. Glasgow, Edinburgh and Inverness, and 4 per half set on the combined Ft Bill / Aberdeen. Clearly no capacity increase in the accessible coach.
I make that an increase of 10 beds of the 112 per half set on the Glasgow / Edinburgh / Inverness, which is 9%, rather than 50%.
But that 9% is frankly irrelevant unless you charge a similar amount per bed as now, which obviously isn’t going to happen. Just on the economics of the couchettes vs proper beds, you canhet 8 couchettes in the space of 6 proper beds, which means to provide more revenue the avergae fare yield for a room of 4 would have to be over 150% of the current average fare yield for a classic room. And hope that other travellers are not put off by the removal of most of the classic rooms, particularly families / groups who would make use of the connecting room facility. I’ll leave others to speculate if that is realistic.