Note: this might be related to https://bugs.linuxsampler.org/cgi-bin/show_bug.cgi?id=315
Looking at the install-data-local Makefile target, ilt looks like DESTDIR is supported for installing the DB in non-global locations. With the workaround of bug #315 that succeeds, however LinuxSampler still attempt to use the global location as follows:
$ telnet localhost 8888
Connected to localhost.
Escape character is '^]'.
LIST DB_INSTRUMENT_DIRECTORIES '/'
ERR:0:Instruments DB directory '/var/lib/linuxsampler' does not exist!
Like already explained in #315, the problem is that there is currently no distinguishing between a temporary installation directory and final installation directory for the instruments DB.