HeII 4686: evidence for an MCV?

No.

The presence of HeII 4686 means two things:

There is a strong enough ionizing continuum shortward of the HeII 228A edge.
There is a line-emitting cloud (or clouds) of gas exposed to the above.

In a recent old nova, the still hot white dwarf photosphere has plenty of photons shortward of the HeII edge --- thus the presence of the HeII 4686 line can be an evidence that the system is a recent old nova. In general, if the white dwarf in a CV is hot for whatever reasons, then the system probably shows HeII 4686 (this depends on the chemical composition of the white dwarf atmosphere, too). However, irradiation of the disk and the secondary by the hot white dwarf tends to produce strong reprocessed continuum, in addition to the line emission, so the equivalent widths of the emission lines tend to be low.

Pure disk accretion seems to have a hard time generating strong continuum in this range --- in a CV, the disk itself is probably never hot enough (unlike in X-ray binaries), plus the disk is in a poor position to irradiate the disk itself or the secondary. The boundary layer may be hot enough, although the energetics of the boundary layer in high accretion rate CVs is a huge question mark at the moment. It, again, is in a poor position to irradiate the rest of the system --- except for the wind. The HeII 4686 line in non-magnetic CVs probably originate in the wind, the same as the UV resonance lines.

It is easy to understand why the Polars show such strong HeII 4686: the soft component (before interstellar absorption) is very strong in the range 50-228A, and the stream and the secondary are favorably irradiated by the soft component. One interesting thought: the azimuth of the accretion region on the white dwarf surface should have a strong influence on how much (and which side of) the secondary is irradiated.

In the case of IPs, the situation is less clear. Although rarely observed directly, we do expect that they, too, have a soft component, since it is difficult not to have L(soft)~0.5L(hard), just from the reprocessing of the latter. The accretion curtain is probably very favorably irradiated, although the secondary may be largely shielded by the curtain or the disk.