Is this a rare/variant character (combining 米 and 団), or not really a Chinese character?
糰 is 米 + 團. 團 simplified is 团. 团 has a variant character 団.

Thus 米 + 団 = 糰.

団 itself is a variant chacter of 糰.

On the other hand it could just be a clever way of writing 米團 = 米团 = rice ball.

Edit: It is written 御飯團 in all instances I can find regarding the ad. It is a private/funny simplification that has no formal recognition.