^That is a good idea, but IMO, you gotta be more subtle than that. An SLE is likely to have the same reaction as I did to those code words, which is: What the hell? Like, why the fuck would I randomly say rhinoceros right before I say something? Why wouldn't I just say it...? So, I would say, do something that serves the same purpose, but without a sort of code system that an SLE (imo) is likely to find silly without being entertaining SLEs only like entertaining-silly, not functional-silly. Functional is functional; silly is silly. Although light-hearted jokes during serious situations (as long as the situation isn't TOO serious) are usually appreciated. And even then, if you have a morbid enough SLE, you can joke about (almost) anything. Although there will be unexpected subjects about which you cannot joke, in all likelihood.
Anyway, I would say, use something more akin to: "Hey, I'm sure you know what to do, but I just thought I'd point out that..." I would say this in a somewhat silly/quasi-subserviant tone of voice which is semi-mocking but not really. (I never really noticed all these things that I do naturally that are super good fits with SLEs). And then when SLE comes back with, "No, we're just going to take I-30, because that leads to where we're going," I'd respond with, "Oh, well, yeah, you know what you're doing. Got it." Or if they were actually wrong, I'd escalate a level without signaling that I'm angry. So it would be like, "No, I'm pretty sure we need to take I-45, because if we take I-30, we're going to hit some traffic I read about in the news." But I suppose at this point I'm basically just telling you to be an IEI, so this probably isn't helpful.
Anyway, you can also sort of tell him what he needs to do to make you happy. So if he needs to use a silly keyword to be clear with you how you're communicating, he probably won't mind (well, actually, he probably will mind, but he'll be willing to do it for twoo wuv



rainbowrainbowrainbow). But again, I'm guessing he won't be too in to that.
What else?
Tone of voice matters a lot to SLEs---you can say some pretty mean stuff without offending them if you say it in the right tone. If you have to say something harsh to them, imagine saying it in such a way that you can finish the statement with this smiley face:

. I mean, don't do that for something major, like, "Your failure to act caused this six month old child to die

." But you can use it for something like, "You know, I *did* kinda tell you to take I-45, so maaaaaaybe you should listen to me next time and we won't be on a parking lot on I-30?

." But make sure not to do this until he's gotten you out of the situation where he fucked up. If you nag him about it during the situation, it'll hurt him. If you joke with him about a mistake after he's already resolved it (i.e., gotten through the traffic, gotten to the destination if it's a long trip, or when you've gotten back home if it's a short one), it won't hurt nearly as bad.
Also, don't be afraid to argue with them and call them out on their bullshit---SLEs are very good at being able to argue with you (as long as you don't trip certain wires) without getting their feelings hurt. It's like sparring. Although that might be more of a guy-guy thing or a general buddy thing than a thing they do with their significant other. But I dunno, most of the Beta STs I know would, I imagine, enjoy a bit of verbal sparring with their partners.