In fact Breitkopf's new Bach edition has somewhat more generous
spacing that doesn't require a tweak.
Overriding LilyPond's very clever spacing algorithm is a dark
art to me. I'm having to do a lot of it right now, but my
approach is unashamedly hacky - but it works for me! Anyway, my
totally hacky solution (which I'm sure will get blown away
shortly) is extremely simple: turn the first crotchet in the
pedals into two notes, but secretly - that changes the spacing
just enough.
pedal = \relative c {\global c1~ | c4*1/2 s8 c4 h a | gis2
gis' | }
c4*1/2 draws a crotchet, but only uses a quaver of time; the
other quaver is filled by s8 (I tried a hidden real quaver
first, but that took too much space), and the other parts move
over a little to match.
Paul