First make a temporary pointer of the size of your pixels that points to the start of the line you want to modify in the buffer.

For a 16-bit buffer:

unsigned short *p = (unsigned short*) ((char*)bd.data + Y * bd.pitch);

Then you can index it like an array to get individual 16-bit pixels across that line, and use Pack and Unpack Color:

p[X] = tcf.PackColor(red, green, blue);

Or to read a pixel:

unsigned char red, green, blue;
tcf.UnpackColor(p[X], &red, &green, &blue);

And for fast blits you can make a lookup table to the current true color format to either table blit from 8-bit bitmaps directly, or convert them to native format bitmaps and then blit them raw.