Is "attentioned" a word?

Solution 1:

"Attentioned" is not a word. "Attention" is a noun, not a verb. You should say:

Send it to [me], to the attention of [my friend staying with me]

For example:

Send it to MKH, to the attention of C.S. Lewis.

Or in a business context

Send it to headquarters, to the attention of the Payroll Department.

Solution 2:

I'd call this a neologism, and it most likely will be mainstreem sooner or later.

Verbification, or simply verbing, of nouns (and more generally, converting an existing word to a different word class) is an important part of language evolution.

For reference I used:
http://en.wikipedia.org/wiki/Conversion_%28word_formation%29 and memories of a wild discussion on word conversion in a Swedish class in high school (those were the days...)