For masters track stuff they're not bothered about frame stickers (and seatpost you won't have a problem with) but re:bars I'd anticipate it'll be another competitor that will likely give you shit for it, not the comms noticing necessarily.
Short anecdote that contravenes the above:
UCI masters world TT in Italy in 2018, they had bike checks set up the day before the TT for everyone to put their bikes through the jigs and give yourself the rest of the day to sort it out before the real deal the following day. Most people were fine, couple of saddle adjustments etc. (my bike sailed through thank goodness), but one woman was getting real shirty with a commissaire. Stamping her feet, shouting, the lot (she was failing the 10cm vertical arm rest to top of shifter rule, using mechanical shifters which is always tricky to stay within 10cm). This went on for ages as she would have had to swap extensions to stay within the rule, then I saw the comm take a little set of calipers out his pocket, measure her Vision carbon basebar for aspect ratio, declare it wildly 3:1 illegal and told her she couldn't race with that set of aerobars anyway, lol.
My most favorite UCI bike check was a masters race in Indonesia - road bike only and I wanted to get on the podium so I wanted to make sure everything was okay with no post race dramas. Asked where the bike check was and got directed to a bored looking official standing in the corner. Tentatively gave him my bike (there was no jig or anything), he gave it a heft and declared that it was probably over 6.8kg so I was fine :D Came 2nd.