Right, so this is the best version of cgminer for modded gridseeds.  However, P2Pool mining is broken for this branch of cgminer-gc3355:
https://github.com/jmordica/cgminer-gc3355

To fix it you need this older branch (that doesn't support freqs above 1150):
https://github.com/girnyau/cgminer-gc3355/

Then you'll build the old branch, but first:

1) From the new branch, copy the driver-gridseed.c, and driver-gridseed.h into the old branch.

2) In the old branch, open miner.h and add the following code beneath line 1048:

extern char *opt_gridseed_freq;

3) In the old branch open cgminer.c and add the following code beneath line 178:

char *opt_gridseed_freq = NULL;

AND the following code block at line 1053:

static char *set_gridseed_freq(const char *arg)
{
        opt_set_charp(arg, &opt_gridseed_freq);

        return NULL;
}

After that, while in the directory for the old branch run:
./configure --enable-scrypt --enable-gridseed
make

And then you should be all set with high frequency support for P2Pool on your modded gridseed!