There is no requirement for a tower ATCO here in the UK either.
Walney Island EGNL has a remote approach controller and only AFIS (Walney Info 123.20 - the Jepp plate lists no other comms frequency) in the tower.
At quite a lot of airports I would have thought a deal can be done with a nearby ATC unit to provide the approach control service (after all it should be able to be a procedural approach in most cases).
This could be done anywhere. NATS (or whichever agency is applicable) would be delighted to provide an approach controller, for the £££.
The problem is the cost.
These things are always confidential so any open discussion is vulnerable to somebody saying "how the hell do you know" but the annual cost of an H24 ATC desk must be well into 6 figures by the time the establishment fixed costs are factored in. Even taking a normal commercial employment scenario you cannot employ a person with anything vaguely resembling a "qualification" (say an on the road salesman or a field service engineer) for much under £100k in total fixed costs. You only have to pop your nose into Swanwick to see that no expense has been spared. Even the potted plants are pretty nice and of course somebody will be employed to look after those, too
The place resembles a bank head office, pre-Lehman. And then you have the massive security, fences, guards, the whole works. And since ATC services would be the only billable product of all that huge amount of stuff, I bet that an H24 ATC desk must cost at least £500k.
A bit less at say Farnborough but not that much less.
The next Q is how much of that £300k (say) will be billed to an airfield doing some private IFR, some AOC stuff, and perhaps a fair bit of instrument training. I have no idea but let's say it is 1/4, which would correspond to an airfield with some busy periods due to training going on (look at Cranfield where ATC tell you to sod off on more days than not if you just turn up and ask for an ILS). That's £75k, and say £40k if you want just 8am-7pm.
Without training activity it could be much less but the airfield will probably try to encourage training to get value for the block payment. I don't suppose the ATC unit will do it on a per-IAP basis because that is a very open-ended arrangement which traditional accountants don't like. It might also make approaches very expensive; e.g. Cranfield charges £30 for an ILS and at that rate many people will not use it.
Any revenue is money for old rope as the controller was already there and the volume is going to be low (otherwise a local ATC would be feasible/probable).
Sure, any ATC activity comes free all the time the system is below capacity (in the same way as the cost to BT of carrying a phone call is only the ink used to print the line on the itemised phone bill) but the accountants you will find in these establishments are not the sort who want to get into marginal costing
The supporting evidence for that is in the 4 or 5 figure annual fee that NATS charge for a radar data feed, which costs them virtually nothing.
One would think that imaginative solutions could be found but the fact that nothing has emerged for such a long time suggests it is a tough nut to crack.
GPS approaches are not the only option, historically. It doesn't cost much to run an NDB and a DME (a few k a year on the maintenance contract) and if an airfield could offer a
published NDB/DME IAP it would attract some valuable training and AOC business. Obviously the actual procedure will be flown using a GPS
but that's not the point.
The "uncontrolled" field is mostly Class E as well, so all traffic in IMC is known. There shouldn't be anything from around 500-2000ft where radar coverage may be patchy.
That is the other bit of the US system which quite cleverly makes it all fit together. Any traffic which is in IMC but not under an IFR clearance is automatically illegal, and the FAA has busted some people for it. Even in Class G (where a "clearance" is impossible) there have been some busts, for flying IFR without an IFR flight plan or something like that.