Apparently when speaking to someone like 明仁, I'd need to speak a special brand of urban-polite keigo called 最高敬語.
Given this, what exactly are the rules for this brand of keigo? As I can only find it on Japanese wikipedia, and my Japanese isn't good enough to thusly read up on it.
No, you don't need it. It's just speaker's choice.
最高敬語 is no longer a different category from normal 敬語, in short, obsolete. In modern language, it consists of some specific words it's just a bunch of leftovers and doesn't have grammar that's generally applicable. So, there are no rules.
Even if you use it to 今上 (the current tenno), it just sounds old fashioned or a kind of joke at most.
e.g.
Among them, 陛下 or 殿下 are commonly used unlike others but at the same time, simply さま is often used instead, though you particularly don't say 天皇{てんのう}さま but 天皇陛下. 崩御 may be heard too.
(When I was writing the slashed part, I was thinking of some 二重敬語 but as a result of adding modern leftovers, my explanation went contradictory.)
