Not really had many more thoughts about this, but have just had the chance of a Clarke CL430 lathe for a very good price, so have been out measuring up the existing CL500.

The base would be ideal for mounting some linear rails on, with two 33mm(ish) flat surfaces with a 68mm gap inbetween.

However using 25mm Hiwin rails and HGW carriages for the X (36mm overall height), and 20mm HGW (30mm overall height) for the Z, leaves only 19mm of clearance between carriage and the bottom of spindle mounting (85mm above flats), which is no where near enough room to fit in enough metal for the carriage and still be able to clear the spindle.
However I don't think it would be too major a problem, as I don't think I'd be doing many large jobs where I'd need to fit the carriage under the spindle.

My current plan is max 70-80mm diameter parts, mostly about 100mm long, but possibly upto 200mm, combined with plans for a bigger bore spindle, so perhaps the CL430 may be a wise purchase (even if it just means I get lots of spares for my 500!)...