I don't entirely disagree with your claim beacon idea, but the "dibs" system is pretty much already included in the border range concept. That's what your area of influence is supposed to represent. And that third party got smart/lucky and snatched a world from the big players. Gotta say I like the emergent storytelling in that ...
I will say though, influence is a ludicrously unbalanced resource, which by extension makes claiming territory a bit wonky. As a democratic government I'm generally running over my influence cap due to Mandates, but as oligarchies and autocracies I'm barely scraping by. Getting influence from rivalries is either a huge risk or nearly pointless - either half the galaxy bands together against you, or you're rivaling pathetic empires that give you 0.01 influence per month. And all that makes expansion super slow, unless you're either a democracy or very effective at conquest. If you're both, you expand way too fast.
Sorry, getting off topic there, but TL/DR: I think the situation that happened to you is annoying, yes, but I'd prefer to keep the option in the game - have used it myself once or twice. As for claiming worlds, I think influence and the cost of frontier outposts needs to be rebalanced soon.