The process by which it was figured out was as interesting as the answer.
There was lots of speculation and many interesting guesses. And Flippy identified it as Japanese kana (as opposed to perhaps more familiar kanji).
Then my friend Mamabunny (who I met at the now defunct Maya's Mom) forwarded the link to her sister, Mikan, who studied Japanese in college (and who writes a great food blog). Mikan was able to provide the answer:
'It basically says "I love" or "I love you" but it's using "degozaru" which an older form of "degozaimasu". It's just an indication of politeness. My roommate likened it to "I love thee."I guess it's something like the French "Je vous aime," as opposed to the more familiar "Je t'aime."
It may or may not be inspired by Rurouni Kenshin, a Japanese comic, in which the main character is constantly polite to the point of being annoying.'
So the verdict on the t-shirt is that it can be warn whenever S. wants. After all, the world could use a little more love.