I have a few ideas, some of which are found in certain context of toons and emails in which he could/should appear (the parts in parentheses referring to contexts):

• "AaAaAa, I'm the lucky number 13!" ("Email #61: MONSTER TRUCK", the part where Strong Mad is about to bash Homestar (and now Homsar) on the head with Marzipan's guitar (after Homestar says his "Brutus" line))
• "AaAaAa, right on! I think I can three times!" ("Email #83: LABOR DAY", occurring after Strong Bad sees Coach Z and before the King of Town; Homsar shuffles up and says the line in response to SB's asking him why can't he understand what he says and he should make sense. He also, of course, is with the crowd at the Labor Day BBQ)
• "AaAaAa, it looks like a Shawshank style!" ("Email #145: MYTHS & LEGENDS", after Strong Sad's scene, Homsar appears in a new scene; he is backed by a message reading, "Another skeptic(?)" (with hot dogs in the background) as he says the aforementioned line about the possibility of the Bear-Shark's existence and Strong Bad responds with, "...said the blue midget Homestar."
• "AaAaAa, I've got patches and am out till last Monday." ("Play Date", Homsar says this line in an extra scene of Homestar calling everyone on the phone; he crosses The Cheat's name off his list and calls Homsar next, saying, "Hello, obscure main guy!". Homsar replies by saying the line listed, to which Homestar replies, as he hangs up the phone and crosses Homsar off the list, "I don't even wanna know...".)

Those lines above are actually what he could/should say if he appeared in those toons.

Oh, yeah, I also thought of a Homsar-type quote that might be spoken by an Old-Timey version of this character, whom is referred to simply as The Homsar (note the "The" part). Here is The Homsar's line: "AaAaAa, I'm a song from the turn of the century!"