Simultaneous marks

[John Galbraith]

I have lots of places in my score where I want two marks at the same place.
In my case, it is a time (but it could be a normal rehearsal mark) and a 
tempo indication.  Right now, I do this:

\mark #`(lines (italic "36:09")
         (columns ((font-relative-size . -1) ,note) (italic " = 120")))

Where I snagged the metronome marking hack from the Lilypond documentation.

I have a lot of trouble with the font specification for the timing mark.
If I put "roman" or "text" instead of "italic", I get fonts that seem, to
me,  random.  I also get lots of "warning: can't find ascii character: 32"
and messages like that.  Specifically, it doesn't like 32, 120, 61, 58 which
are spaces, "=", and ":".  Also "x", but I have no idea why it is
even looking for that character.  So, it kind of works, but I get
lots of these warning messages and sometimes the font comes out weird
(like the first mark will be roman, the next will be the time signature
font, and I have even seen it come out using the basic feta font characters
instead of roman).

Also, I would like to be able to box the timing mark without boxing
the metronome mark.  I found the box hack in the Lilypond
documentation, but it seems to box the whole composite mark, not just
the part that I want boxed.  If I could get to different marks at the
same time, I could change the mark properties in between them.  With
this composite mark, though, I need to be able to box only part of it.
Any hope of doing this?

By the way, I like the metronome mark in the \mark command because I put
them in a global part that is used everywhere, so it is hard to associate the
metronome mark with a specific note in a specific part.

Thanks for everything,
        John