To do so, I'd have to bring the existing tundish down from about 1m high
to 30 cm off the floor (is that in itself allowed?), and add a tee into
the 22mm copper below the tundish, connected to a short vertical run
into which the 0.5" plastic drain hose from the dehumidifier would be
tucked. Diagram below.


ISTR there's regulations about the distance from the PT relief valve down to the tundish.
Let me look up my megaflo installation manual...

G3 GUIDANCE SECTION 3.9
The discharge pipe (D1) from the vessel up to and including the tundish is generally supplied by the
manufacturer of the hot water storage system. Where otherwise, the installation should include the
discharge pipe(s) (D1) from the safety device(s). In either case the tundish should be vertical,
located in the same space as the unvented hot water storage system and be fitted as close as
possible and within 500mm of the safety device e.g. the temperature relief valve.

--
Ron