To know if you can make traditional after-tax contributions to the plan, go to the contributions tab where you'd select a % of income for Roth and/or pretax. If mega backdoor roth is set up, there will be a 3rd line for after-tax (not mentioning roth).
:( ... no such luck... only 401k and roth 401k lines (fidelity website)
is there any reason that a company would not allow this? ... seems like it does not impact them/cost them anything ...
There are a few reasons, and #1 is pretty legit:
1) Concerns about passing ACP testing. Any company that is a bit close on their testing is not going to consider enabling traditional after-tax which is necessary for the mega backdoor. Typically only high earners are going to take advantage of it and passing the ACP test can already be a problem for some companies such that they don't want to add to their problems. When I was talking to the Fidelity expert on this he said most demand was coming from Silicon Valley, I assume at least part of that is due to the high salaries there.
2) Awareness and concern on this is very low. I would talk to a variety of people making 250k+ salary and they'd be blown away that someone could save 18k let alone be interested in ways to save beyond that.
3) Its a fairly new concept
Many 401k committees struggle with basic things and just getting decent index funds can be a challenge. If this is really important to you, you'll want to interact directly with your 401k committee (and maybe ask to join) in order to see forward progress.