I've reached Tier 2 with my Combat Evolved marine from 2019, and I'm sure I could do better if I did it again, as I've looked at the armour a lot more since then.
Modded content alas wouldn't get past a Tier 1 deployment, unless the modifications still fit into the specifics of Tier 2 (ie. "Costume may have paint details or decals that simulate armor graffiti or personalization but they must reference items, groups, or historical incidents that exist within the Halo Universe. For example Phrases such as “Remember Reach” or UNSC or ONI related logos or markings. Generic phrases like “Follow Me” or “Come Get Some” and generic logos like the “Skull and Crossbones,” “Smiley Face”, or Animals are acceptable but references to other Fictional Universes are no longer acceptable for Tier 2.")
Don't forget, once you've deployed your armour, you can change it afterwards and not lose deployment! So if you wanted, you could hit T2 with the marine armour, receive your deployment, then make the modifications to your armour to match the mod. It just means you couldn't apply for deployment again with that armour, unless you change the modifications back.
"In game" still requires that the content be developed by Halo developers, or at least added in to the game by the developers.
