Not really Bolty,
ETOP's makes allowance for single engine diversions within its rules, but it is not simply about flying on one engine as you can see from the critical fuel scenarios (item 1) as follows.
ETOPS fuel requirements in additional to the normal fuel requirements require that 3 critical fuel scenarios are taken into account, and that each of these 3 scenarios permit a low level diversion to a suitable diversion airport with the normal holding and approach allowances intact.
The scenarios are :
1) Total pressurisation failure.
Immediate descent and 2 engine cruise to diversion at 10,000ft
2) Engine failure.
Descent to stabilising altitude and single engine cruise to the descent point for landing at the diversion airport.
3) Engine failure and simultaneous pressurisation failure.
Immediate descent to 10,000ft and single engine cruise to the descent point for the diversion airport.
Invariably of course from a fuel critical scenario case 1 is the most critical, and is covered by the normal ETOP's planning rules. However multiple failure case 3 is also covered and would not as you suggest result in not being able to make your diversion airport, unless you had failed to comply with the ETOP's planning rules.
As you say the single engine situation would result in a descent to a stabilising altitude that is mass dependent. Probably somewhere in the low to mid 20's (x 1000 ft). At these altitudes / levels the one engine has much less bleed demand to operate the packs and pressurisation is not normally a problem. I am a bit lost as to Aerotechs point, since if both engines fail ETOP's is a bit of a moot point !