[TYPO3] setting TLO _DEFAULT_PI_VARS dynamical

[t3developer]

Hi list,
i have installed the timtab extension. the blog is only accessable for 
logged in users.

so i want to set the postform fields firstname and email with the values 
of the user which is logged in.

the postform is done with ve_guestbook.

i try this but it doesn't work:

plugin.ve_guestbook._DEFAULT_PI_VARS.firstname = TSFE:fe_user|user|name


how can i solve this problem? setting piVars by typoscript dynamical?

thx Frank