Strings in figured bass

[Gilles]

Hello.

I've just installed 2.4.2 because "Regression tests"
(http://lilypond.org/doc/v2.4/input/regression/out-www/lily-347603903.ly)
showed that strings now appear in the output (there was a blank instead
in v2.2.6)

I thought I could use that feature for some missing support in the figured 
bass notation.  Namely, the piece sometimes contain a "x" (cross) before 
the figure.
I tried 

  <"x"4>

but, although the string is "glued" to the digit, it appears in the output
as if there was a space between them (hence the "x" is printed above the "4").

Whereas, with

  <"x4">

the layout is right but the digit is printed with font and size different
from the other figures.
What would be a workaround?

Thanks,
Gilles