Custom Cages are eligible for FIA or MSA certification. See below - excerpt from:
http://www.customcages.co.uk/roll-cages/upload-dvd-sample.
To compete in any race or rally your roll cage will need to comply with either MSA or FIA regulations. (Effectively, the MSA is governed by the FIA so if a roll cage design complies with FIA regs, then it is also eligible for MSA events). There are some instances where the FIA allow certain design features where the MSA do not (and vice versa).
A roll cage design must comply with the MSA/FIA technical regulations published in the MSA Blue book. It is the competitor's responsibility to ensure his/her roll cage complies with the technical regulations appropriate to the event they are competing in.
The certification process generally allows for a lighter roll cage because it is a bespoke design where the diameter and thickness of each tube is optimised to achieve the overall strength required by the FIA/MSA rules.
In order to compete in MSA/FIA events every vehicle, which has a certificated design of roll cage, should have a copy of a valid certificate to present at scrutineering. Custom Cages has certified the majority of popular roll cages. All other roll cages comply with MSA/FIA regulations and receive a Roll Over Protection System Description (ROPSD) for competitors to present to scrutineers.