As said you can retrofit DPFs. This will give you a Reduced Pollution Certificate, aka an RPC. This was the case for trucks too.
Note the word was. I know when I was involved in transport, VOSA stopped giving out RPCs for lorries, probably due to the ever moving goalposts.
A shame it has gone that way. Small wonder as to why new car prices are so expensive.
They've changed it to a Low Emissions Certificate (LEC) now. These are special rules for HGVs, motor homes, vans and 4x4s and I couldn't find anything similar for cars.
TfL have a list of approved filter suppliers to qualify for an LEC, maybe speak to one of them about your idea, or ask VOSA themselves.