We've got one problem machine that has about 90GB in one DLE, and our tapes
are 100GB LTO1s.

Of course, once a cycle, the server decides it can only write maybe 5 DLES
out of 150+ to the tape, because one of them is 80GB+, and so all the other
dumps end up being reported as

  FAILED [dumps way too big, 1 KB, must skip incremental dumps]

which afaict means that amanda doesn't even attempt to back up this DLE
because it knows it's not going to fit it on the tape.

But I've got a ton of free space in the holding disk.  What I want to know
is, is there an option to tell the server to do the dumps anyway and hold
them, so later I can come along with another tape and flush them?

In general, we only need one LTO1 a day, this particular DLE is a freak
statistic well outside the standard deviations of the other DLE sizes, so
doing a flush once every cycle is more appealing than allocating two tapes
per run.

At this stage, splitting this DLE into smaller chunks is also not an option,
in case you were wondering.