Battlerider and Brilliant Lances both assume the use of nuclear missiles. While the Imperial Rules of War may not apply anymore, I found it a little odd that rules weren't provided for any other munition, since I figure the majority of missiles produced in Imperial times must have been conventional.
It's probably exactly as you think. It's an oversight by GDW.
The TNE era rulesets brought about quite a few fundamental shifts in the technology assumptions of the universe (most notably a shift away from "grav plates" towards heplar but there's others). Due to thinking of GDW about missiles (they should be somewhat small, space combat occurs at super-long ranges requiring the handwave of grav focusing for lasers, and so on), missiles that use blast effects against their targets aren't very useful - they move too slow and are too easily shot down (this doesn't take into account that many smaller ships don't seem to have any way to shoot down missiles, oops). So the only relevant missile type for space combat would be pumped det-lasers which aren't really missiles but more of a stand-off platform for laser shots.
I've always figured that TNE-era had a "we have always been at war with eastasia" type retcon where the Imperial Rules of War meant you couldn't use nuclear weapons as direct blast weapons - using them to pump detonation lasers is fine. Using the same detonation laser in atmosphere for EMP or nuclear blast effects is a no-no.
there should be other ways of creating intense energy releases sufficient to pump a laser in the next few decades (real world)
Actually that might be a more elegant solution. Perhaps around TL11 or something they do come up with non-nuclear high efficiency methods to get a similar effect to detonation laser performance without using nuclear detonations. The development is a technology driven by the Imperial Rules of War. At TL11, they're much less effective than det-lasers and more expensive, however they're not nukes and they have a reasonable performance. At TL12, less effective but getting better and become cheaper and so on. At TL13 they achieve near-parity in effectiveness but not in cost. By TL14 the non-nuclear versions are more slightly effective than nuclear pumped versions but remain expensive and require TL14 industries to build them. At TL15 and beyond, they become increasingly more effective and increasingly cheaper but never as cheap as a nuclear one.
Within a year or so of the Rebellion starting, the unprecedented ferocity of the civil war has meant the former fleets of the Imperial Navy has shot off most of its non-nuclear laser-missiles. With the "one true Imperium" fighting each other, everyone realizes that making these non-nuclear laser missiles isn't a good use for production except on the few TL14+ worlds (where it competes with other wartime production) and such missiles are reserved for primary naval fleets. Secondary fleets don't have enough missiles to fill their magazines. Everyone else can't get them at all.
Of course, with the lapsing enforcement of "rules of space" (as described in
Hard Times) everyone knows that nuclear pumped det-lasers are undeniably effective, and a TL10 nuclear laser is as good as a TL13 non-nuke one, a whole lot cheaper and can be fashioned fairly easily even in a "garage" workshop at TL14-15. So secondary squadrons start using nukes. Raiders and deserters do the same (cut off from conventional supply). Merchants operating outside of the Safes start doing the same.
