As everyone knows, every action has an equal and opposite reaction thus not being able to keepsake an untrimmed cape is the reaction to your, getting a 2nd cape, action.
Judging by the mention of Mod Kieren, I'm going to assume the OP was talking about OSRS with this question.
In OSRS, you can keep your untrimmed skillcape by dropping it on the ground, and then telegrabbing it after you earn your next 99. Assuming you can find a way to remember to do this for every other skillcape you earn, this is how you can keep your untrimmed cape.
Would love this but alas I got my first 99 in fishing before keepsake or customisation was a thing and you was seen with what you were wearing it wasn't till 2 years later by which time I had 3 99s that keepsake was a thing and people could save their first 99 capes and keep them untrimmed.