I know of at least one individual who customised/preserved their ritual knives, but now I am told this is not considered acceptable. What quantifiable difference is there between this and a Magi's quarterstaff? If I were paranoid, which I'm not, whatever people are telling you, I would be inclined to think that the difference is that there is an artefact available for one, but not the other. (but then there're Grove quarterstaves, which have an arte version but can be customised)
Regardless, this definitely seems an inconsistent application of policy. While an update to the HELP file would work, I'd be lying if I said I didn't want my customisation, which I put a lot of effort into.