Here are a few stories from the Eunuch archives about guillotines, starting with my favorite one. Enjoy. (Oops, didn't realize you were looking for off-site stories. If I come across any I'll post them here.)

http://www.eunuch.org/Alpha/C/ea_191724cock_gam.htm
http://www.eunuch.org/Alpha/P/ea_33113the_pane.htm
http://www.eunuch.org/Alpha/U/ea_193723unmanned.htm
http://www.eunuch.org/Alpha/M/ea_02941miss_col.htm
http://www.eunuch.org/Alpha/C/ea_132104cock_mod.htm