<<What would be a likely DH - 300ft QFE perhaps? (We used to operate to 200ft in the UK many years ago. >>
In the UK PAR talkdowns usually continue to 1/2 nm from touchdown - just over 150 ft height - but can go to touchdown when required. I suspect that if most pilots saw nothing at 1/2 nm they would pull-up pretty smartly.
The question of descending out of cover behind high ground is interesting. It happened long time ago with an aircraft inbound to Northolt when it descended below the PAR glidepath. The resulting "emergency" climb prevented a major accident. Radio communication should be OK until some while after the a/c dropped out of radar cover at that short range.