It is "Not a good idea". The Lotus cannot generate enough rear brake bias so adding the proportioning valve to the rear is useless. The only way to get the valve to work would be to increase the rear brake bias to the point that you had too much. If you keep the stock brake master, the only way to accomplish this would be to install larger calipers on the rear. You would not want to install the valve in the front brake line.Thanks.
I am trying to figure out whether a valve is "a reasonable low budget solution, not as good as a bias cage" or just "not a good idea".
Looks like it is the former.