Looks like there are two possible solutions:
--soname-synonyms
If your custom allocator functions are still called malloc() and free(), use the --soname-synonyms
command line parameter to tell Valgrind to look for these functions in your preloaded library. It can then automatically intercept calls to these functions. Example:
--soname-synonyms=somalloc=mymalloclib.so
See http://valgrind.org/docs/manual/manual-core.html#manual-core.rareopts for details.
Client Requests
If you have a more complicated custom allocator (with an API different from malloc()/free()), you can use special "client requests" in your malloc()/free() implementation to tell Valgrind about the structure of your memory pool. See http://valgrind.org/docs/manual/mc-manual.html#mc-manual.mempools for details.
I haven't tested either of these solutions :-) so would be nice to hear whether it actually works for you.