Is it somehow possible to tell Lilypond to look for the .ttf file in
the working directory, ie. the directory in which the .ly file is
that is being compiled?
Yes. Here's some documentation from the current development version;
the two mentioned scheme functions, however, are present in 2.20, too.
Finding fonts
-------------
In addition to any font already installed on the operating system,
more fonts may be added to the ones detected by FontConfig (and thus
available in LilyPond scores) by the following commands:
#(ly:font-config-add-font "path/to/font-file")
#(ly:font-config-add-directory "path/to/directory/")
Both commands accept either absolute or relative paths, which makes
it possible to compile a score on any system by simply distributing
the relevant font files together with the LilyPond input files.
For your case it should be thus sufficient to add
#(ly:font-config-add-directory ".")
at the top of your main `.ly` file (assuming that you call the
`lilypond` binary while being in this directory).
Werner