It does not currently appear possible to make attractive cross-measure tremolos. In other words, tremolos between 2+ notes are restrained to fitting inside a single measure. Though the same tremolo can hypothetically be placed on multiple staffs and then tied, this is difficult to understand for a musician.
Rather than treating the tremolo lines that beam 2+ notes as ornaments such as the accent or staccato, treat them as lines such as the 8va line. This would allow tremolos to last for any duration of time, and would also allow more control over the size and location of the tremolo marking.