In fact you can, though not through the interface. Use gconftool -s/apps/galeon/Advanced/Network/user_agent --type=string "Mozilla/4.0 (compatible; MSIE 5.5; oops, wrong! it's:) Mozilla/5.0 Galeon/0.12.5 (Linux i686; U;) Gecko/20011012", for instance
In fact you can, though not through the interface. Use gconftool -s /apps/galeon/Advanced/Network/user_agent --type=string "Mozilla/4.0 (compatible; MSIE 5.5; oops, wrong! it's:) Mozilla/5.0 Galeon/0.12.5 (Linux i686; U;) Gecko/20011012", for instance