Hi Mike,
Cassette water pumps seem to have had mixed reviews over the years and although the logic is sound, whenever I've had to do mine I've looked, shrugged and just put back the standard Lotus/Ford parts. I can see the advantages all right, it just seems expensive for a job that you usually only need to do at rebuild time.
I don't know about the idea of a hole in bulkhead. It sounds ok but I'm not sure what access you'd get because the engine block/front cover is quite low in the "V" of the chassis ? I suppose it depends on the design of the revised front cover and how much room the cassette unit needs to come out ?
Despite the workshop manual starting off the "how to replace your waterpump" section with "1. Remove engine compartment lid" followed by "remove engine & gearbox", in practice there is enough room to get the cover off to replace the pump with the engine still in the car. It's tight but it is possible. But much easier for Rob to do his now, while it's all in bits....
Brian