With version 1.2.1, I tried to use xml-rpc to subscribe a RSS feed but I failed.
There was no response. But I tried again, I got a message "Feed already exists in the database.".
So I added a few debug code to subscribe_to_feed() and update_rss_feed() in functions.php.
And I found that fetch_rss() of rss_fetch.inc was not called by update_rss_feed(), line 280.
When I added the following code at the beginning of functions.php, fetch_rss was called and I got a message "Subscribed successfully." from xml-rpc. It worked fine.
require_once 'magpierss/rss_fetch.inc';
I think you'd better add the code somewhere in functions.php. (I don't know where is the correct place though...)
Testing environment:
tt-rss 1.2.1
CentOS Linux 4.3
PHP 4.3.9
Apache 2.0.52
MySQL 4.1.12