[TYPO3-mvc] fluid using assigned variable inside of an data relation lookup

[Domi]

Hello,

I would like to know if and how it is possible, to assign inside of my 
controller a string, which should be used inside of an object lookup.

Here the sample:

{mail.toUser.uid}

Now I want to switch between toUser and fromUser through my controller:

$this->view->assign('myUser', 'toUser');

I would like to know how I could write this in fluid now:
{mail.{myUser}.uid}

I would like to avoid many if/else causes.

Thanks,

Dominic