My team has a variety of equipment. Some is "pooled" equipment (portable base stations, portable repeaters, etc.) but others (HT radios and such) are assigned. I currently keep track of all the serial numbers, etc in an Excel spreadsheet which I could upload into the documents section of MM. But I see several other possibilities for a solution to your question, using my team as an example. All of our team members are licensed amateur radio operators. We have to use our amateur call signs while operating on ham frequencies, so I created a box in Certifications for that license number. Several of us also have Automatic Position Reporting Systems (APRS) so I made a Certification box for that which contains that ID number. We also have tactical call signs, which are used on both commercial frequencies and amateur frequencies (in conjunction with our ham call sign). These are also our personnel ID Numbers, which appears at the top left in the Member information page. There is also the Radio Field Name, which I designated as Radio #. I can then put the radio model and serial number in there. There is also an empty box that show up under our equipment sub-heading for each member called "Other:_______________." Could that work for you?
It sounds like your CERT#s are your tactical call signs, but probably not your Member ID Number. Do your CERT numbers change from one incident to another or are they permanently assigned? If they change for each event, then you could look at the CERT number as a team assignment ID when you are in a Mission. If they don't change, I think your best bet would be to use the Other box if you want it listed in Equipment, or just make a new Member Certification box.