If one goes to the Store, you can click on any 'mech chassis and click 'Open in MechLab' in the bottom right:
This will allow you to pretty much edit and modify the 'mech in any way you see fit, similar to the smurfy mechlab minus the ability to save. From my playing about it doesn't seem to add C-Bills to your cart either, so you're not limited by your warehouse or credits. You can even play around with the camo spec.
EDIT: So it does throw up a warning when you go over your c-bill balance, but you can still edit as per normal nonetheless.
Edited by Dingo Red, 02 June 2015 - 03:33 PM.