ATM, you can only use columns of AD_User table for the user mail signature https://idempiere.atlassian.net/browse/IDEMPIERE-1899.
I have the requirement to put the greeting.
I have updated the parse method from MMailText in order it calls Env.parseVariable.
And now, I'm able to use this kind of mail text :
, I committed a modified version, reviewing your patch I was just concerned that mail text has some default formatting that Env.parseVariable doesn't have - so integrating your patch can potentially break mail texts used elsewhere trusting in such default formatting.