I remember seeing a thread on the old bobs or another site about how people used to seal motors using the seals like your talking about, until people figure out how to order the same seals online somewhere and copy them, so they basically figured a way to cheat then put a new seal back on it. There really is no foolproof way to seal a motor and keep it that way in my opinion, unless you were to buy a box of those small padlocks maybe where you are the only one with a key and use a steel cable between the block and sidecover like you are talking about doing, then impound the motors at the end of every race day, and let people pick them back up at the beginning of the next race day. You could engrave each persons name or team name into the block to mark the engines so you know who's is which one. That seems like the only real way to keep everyone honest, and even then people will still find a way to tinker with the engines at the track if they get a chance to. There will always be those people who will go to ridiculous lengths to cheat and find a way to beat the system no matter what you do. Even if you seal the blocks, what about the heads?
You might be able to go to a paint store and mix up a one-off odd color and mark the bolts on the engines with paint like the tech officials do at some of the big races, that way you can tell if someone has been into the engine or not. I still dont see a foolproof way to seal engines, but it would be a great idea to keep a level playing field if someone could come up with a foolproof way to seal them. I dont see the little numbered or lettered laser etched cable seals working for very long before someone figures it out....