You may suggest it to Markdown mode. I think this is a terrible idea for
Org mode: syntax should not be optional.
But I'm not talking about org-mode !
I'm talking about orgtbl minor mode.
That minor mode is supposed to work in other modes (that's the whole purpose of its existence),
and so it should be able to adapt to the table syntax of those modes/notations.
Yes, markdown-mode may need to do something to use orgtbl-mode.
The problem is that the two functions are not adaptable because "+" is hard-coded in them.