Custom key signature at line break

[Janne Uusitalo]

Hi,

if I typeset a key change from b major to g major, f remains sharp and
LilyPond prints out the naturals necessary for c, g, d, and a. Makes
perfect sense, but the standard I'm required to follow has it that an
extra # for fis should precede the group of naturals. Is there a way
to achieve this? I get close by creating a custom signature with the
right set of symbols, but as the change occurs at line break, I'm
stuck figuring out how to print this custom set at the end of line 1
and only that of g major at the start of line 2. Any helpful ideas to
share?

BR,
Janne