There are commercial wood end grain sealants available. Melting wax in a double boiler is ok. Thinning out white glue with water and smearing it on works. Thick paint is better than nothing. Anything that reduces the woods direct exposure to wind and helps the moisture in the wood to leave slower is of benefit. I guess with those river logs you have total saturation and something different from simply green wood. I don't know if that makes wood more or less susceptible to cracking compared to being freshly cut? Is there increased movement?
Generally when sealing wood, you are not going to completely prevent cracking, minimize it at best. I think what I would do with those river logs if in a hurry, is have the wood milled and sticker it without sealing to begin with. Maybe it is better to lose some of that heavy water content somewhat quickly. Keep a close eye on it and at the first sign of cracking or rad warping, then seal it. You won't suffer anything by not sealing it right off the bat. If it is going to crack heavily, it will do that with sealer as well. Expect cracking along and near the pith. Keep the sun off it and also exposure to drying winds, if you can. Ideally, I think the best plan if time allows, is to leave the wood in log form for a year, then have it milled. Wood that loses it's initial moisture while the wood is locked in place, so to speak, in the log, tends to be more stable after being milled and further dried. In the log, you aren't really expecting to lose so much water, just add stability. I think I would seal it and the same about preventive exposure to sun and wind. After you mill it will be when the major drying occurs. The best way to prevent cracking and have the most stable material is to have the wood quarter sawn. Good luck!
