My family did not decide on a memorial for my brother until after the internment 2.5-weeks following his death. As I reply to people who sent cards & calls & flowers, would it be okay to mention that a memorial fund was selected if they want to donate?

Mention, yes. Ask for a donation, no.

It would be more appropriate on a thank you note rather than a card. Then it's merely information.

A thank you note is presumably being sent because they've already recognized your brother's passing in another way. So, no, don't bring up the memorial in the note and don't expect more from these people in the form of recognition of your brother's passing.