No, there aren't any and I doubt this'll change any time soon, especially considering how volatile the altcoin world is in terms of which coin is trending at the moment. You could try and dig around Unicode characters for symbols similar to whatever coin you want to represent or just link to small icons (like some do with custom emoticons) in your posts. If you're familiar with Javascript, you could write up a few userscripts, which add additional buttons in the "Post reply" page for quick embedding of such symbols / images.
EDIT: Aaand I decided to make a userscript anyway: https://github.com/mprep-btc/Bitcointalk-Post-Iconizer
Thanks a lot for your reply and also for creating this userScript. Although a combination of various keys to produce these symbols may be a bit annoying to many but given the huge numbers of Altcoins that we have,we obviously can't have a code for each of them. EDIT: Aaand I decided to make a userscript anyway: https://github.com/mprep-btc/Bitcointalk-Post-Iconizer
