Underlining fingering instructions without loosing positioning

[Morten]

I'm trying to have underlined fingering instructions in Lilypond v. 
2.12.3. - On a button accordion this would mean that the "helper rows" 
(I don't know the English term) should be used instead of the main rows.

I've seen different posts about this, but in every case, the solution 
has had to do with using \markup. I've been using something like:
^\markup{\finger \underline "3"}

This approach works fine in simple cases, but if the music contains 
slurs, the spacing between the slur and the fingering instructions vary. 
This doesn't look very good, and it makes the fingering harder to read. 
The example below shows the problem:

c'4-1( d-2 e-3 f-4 g-5)
c,4^\markup{\underline \finger "1"}( d-2 e-3 f^\markup{\underline 
\finger "4"} g-5)

The \finger apparently ensures the correct font, but is there some 
command to make it "behave" or position itself like a fingering instruction?

Regards,
Morten