After giving it some thought, I think you should indeed do that. For Lemmy AND Kbin and more.
tl;dr: Advertising the existence of kbin and lemmy to random Reddit users is exactly what you want to do if you want to go against Reddit, and r/place is an excellent way of 1) telling people who don’t know about it that these platforms exist, and 2) showcasing the vitality and size of the communities on these platforms

The major objection is that going to r/place gives Reddit the engagement and numbers they want for the IPO, and I think that’s a compelling point but I don’t think it’s as obvious as the people making that point seem to think. The idea of “don’t go on Reddit to protest Reddit, that’s just helping Reddit” has some “But you live in a society, curious” vibes to it; I think the question of whether to protest vs abstain and how to best protest is always going to depend on the details of what you’re protesting or abstaining from.

In this case I think Kbin and Lemmy users should put their names on the r/place board according to the following reasoning:

• The argument that you shouldn’t go on r/places is essentially saying that the best protest against Reddit is people leaving Reddit, which I agree with

• Like all protests however it’s not that impactful if it’s a few isolated people doing it, you need to find a way to have users do it en masse. Coordination is key.

• Same thing for going on Kbin and Lemmy and others - these platforms become good if they have enough users to sustain vibrant communities, they rely on network effects.

• r/place as an event is a showcase of a community’s coordination. It both requires a community to be large and well-communicated and it gives a very practical, visible way of advertising that coordination to both rivals and random observers (there’s a paper out there proposing that this is why music evolved btw, hmmm that’s pretty cool)

• what ultimately made me decide to post this is going on the thread for r/place’s first day. Look at the conversations, this is exactly what they’re doing: discussing the communities participating, commenting on what they draw and explicitly talking about what it means for those communities’ size and coordination

• These comments also included people asking “why fuck u/spez ?” and “the only reason I’m still on Reddit is that there aren’t any alternatives”

• This means there is a pool of normie users who aren’t aware of the protest, but are following r/places, and the “fuck u/spez” movement is effective in bringing their attention to it

• By the same token there are tons of users who aren’t aware of existing potential Reddit alternatives (one of those comments got “Lemmy” as a recommendation in replies and said “interesting I’ll check it out” - they legit hadn’t heard about it).

In conclusion:
Advertising the existence of kbin and lemmy to random Reddit users is exactly what you want to do if you want to go against Reddit, and r/place is an excellent way of 1) telling people who don’t know about it that these platforms exist, and 2) showcasing the vitality and size of the communities on these platforms.

Now in practice I don’t know that these platforms actually have the size and coordination to showcase that on r/places and that’s fine, clearly a huge percentage of people here believe that boycotting Reddit entirely is more effective or more convenient. But if the question is “which hurts Reddit more, promoting Lemmy/Kbin on r/places or avoiding r/places”, I’ve come to believe the answer is the first.

EDIT: oh right another objection I saw was “but the admins will just erase it”, and there again look at the comments on r/place. Clear streisand effect on the guillotine, if there’s stuff for lemmy/kbin/squabble that’s visible enough and admins erase it it still works fine from a comms perspective.