Disclaimer up front: I am not a nurseryman
I agree with @Heliostar, I have almost never seen an appealing graft for JM bonsai. Maybe this is because some are so good you literally “don’t see” them…but I believe if this were to happen it is far more likely the exception than the rule. Most that practice this hobby/life choice called bonsai try to immediately rid any nursery tree of grafts as soon as they receive the material, not the other way around.
There are several aspects that make grafts more or less successful from an aesthetic point of view. You should try to match the growth rates of the scion and rootstock as much as possible. A slow growing cultivar will not do well with a fast growing one and you will increase the mismatch in size of the trunk at the graft union. Second, match the bark texture and color as much as possible. Typically this is difficult for seedlings as they don’t have their mature bark yet, but do your best. Finally, try and match the cambium layers as closely as possible and afterwards restrict movement as much as possible to reduce scarring at the union.
Since you have interest in using your rootstock outside of bonsai it never hurts to have a bit more information about techniques. I believe that the way this is done (most successfully) in the nursery industry is to chip bud the scion onto the root stock. I assume this is the method of choice typically because you only need as little as one bud of the scion to grow an entire tree, but I also think that this method would lead to the least amount of scarring.
https://www.rhs.org.uk/advice/profile?PID=400
This resource below is a wealth of information, even though many of the techniques are not going to be applicable to bonsai. It covers many different types of grafting and budding.
Finally, just experiment and see what you come up with. Maybe they will fail, maybe you will come up with a new method that we all forever refer to as the risco method. I have always thought about trying to use root-grafting techniques, frequently used with tridents, above the graft on some interesting, yet weak-rooting JMs. The thinking being that having multiple root-graft unions all around the trunk might give the grafts a knobby, “interesting” type look instead of a “flawed” look. Someday maybe I will get time to try this.