An update on that if anyone is interested...
Here's how it worked:
Go to the immigration office in Sop Ruak. They will stamp you out and they also ask if you have at least 10,000 baht in cash and an outbound air ticket. (On arrival, they did not ask to see that).
After getting stamped out walk out the back door which leads to the boat dock. The boat across to Lao costs 70 baht if there are at least three passengers and they run on demand.
Once on the other side, you disembark and walk up to Lao immigration. The first window checks passports and hands you an arrival form, which you complete, then hand it to the next window and then pay at another window. The visa cost varies by country - for Americans it was 1,500 baht.
You can then go to departures, get stamped out and return to Thailand to get stamped in. The boat back to Thailand costs 150 baht. BTW, there is also a small duty free shop inside the Lao arrivals hall with decent prices on spirits and wine. If you have USD, use those as they were doing 37 baht/dollar for purchases.
Once back on the Thai side, you just need to go through Thai arrivals and you're stamped back in. The immigration officer advised and confirmed that they will only allow this twice each year.
Seems easy to me if you need to do a border run and maybe faster than going through Chiang Kong and across the bridge there, at least until Tachilek reopens, which of course is faster and cheaper.