How can sex ratios remain Fisherian (1:1) in species where only the dominant male gets to mate
Fisher's principle applies to such cases as much as it does to species where only pairs mate.  Consider a species where a successful male has exclusive mating with a harem of 20 females, and for each such male, 19 other males are not able to mate.  A female has 100% chance of mating, and a male has a 5% (1 in 20) chance of mating. Assume a female has two offspring. 

In this scenario, an equal sex ratio would mean having a female offspring would lead to an expected number of grand-offspring of 2 (100% chance of 2 offspring from that female).  Having a male offspring would also lead to an expected number of 2 grand-offspring (95% times 0 offspring plus 5% times 20 harem females each having 2 offspring).

What would happen if the ratio of births was off from 1 to 1, say 5 females were born for each male? Then expected number of grand offspring would be for a female is still 2, but for a male it is 25% (chance of taking a harem of 20 females against the 3 other males born alongside those 20 females) x 40 (20 harem matings x 2 offspring) = 10. Hence with a lopsided sex ratio in this situation, having a male is much more valuable genetically than having a female.  If a mutant arose which produced more male offspring in this imbalanced situation, it would have success until the population sex ratio became close to 1:1.