Extending value of note in tremolo

(A few bug reports may come out of this.)

Whilst you can decrease a note value with a tremolo and retain the latter, you can't do the same if extending it (open attached score and change value of note to semibreve).

I wondered what the opinion of this was generally (especially if it exceeds bar), and also if there was a tremolo between notes (opposed to on the stem, as featured in this example)?