I have Xubuntu 11.10, xfce 4 desktop.

I have copied a bunch of .ttf fonts to ~/.fonts, ran fc-cache -f
The fonts are not listed in LibreOffice 3.4 fonts dropdown.

Then I tried creating myfonts directory:
sudo mkdir /usr/local/share/fonts/myfonts, then
chown root /usr/local/share/fonts/myfonts/*.ttf, then
cd .. then
fc-cache ...and fonts weren't listed, so I tried
fc-cache -f ...and fonts still weren't listed.

I need my truetype fonts, which I have modified for my special purposes.

I would be grateful if someone could tell me how to install fonts in xubuntu 11.10 -- and get them to be recognized by Libreoffice. They're not seen by gucharmap, either.

Did you try logging out and back in after installing the fonts? According to the Ubuntu wiki (see here), some programs may require that before they'll see the new fonts.

Agreed, restart your system, that should work. I use the same setup.