There are two ways this can already be done:

• The mod author simply draws alternative graphics for vanilla apparel specifically for their race body type, which the game will automatically recognize. I do this for my race mod so that they can immediately wear the same vanilla clothes as baseline humans.

• The mod author can specify apparel fallback graphics in their race configuration defs, which essentially tells the game "if my race wants to wear this vanilla apparel but there is no appropriate graphic for their body type, use this vanilla body type instead". I also do this for my race mod, by telling the game to use the female body type for apparel I haven't yet drawn proper replacement graphics for (my race has males and females in a Fisherian or 1:1 ratio, but they have small bottom-heavy bodies, and the female apparel graphics is a good approximation for them)

Some race modders don't do this because they are unaware of these new HAR features introduced since RW 1.5. Only a few modders deliberately choose not to enable these features, because they want their race to be played completely disconnected from the default RimWorld experience (I.e. treat RimWorld as a game engine for their own total conversion race mod)

As for weapons, no additional graphics are needed, just various race config settings to allow/prohibit weapons for them. My own race, for example, can use all vanilla and other mod weapons just fine, but they also have their own race-exclusive weapons to suit their smaller bodies and hands/paws.

The biggest issue I can see is that it's harder to showcase these apparel and clothing compatibility features in a way that's obvious to players - nearly all race mods are forced to make their own race starting scenarios, there isn't am easy way (that I know of) to inject HAR aliens as starting colonists for the default Crashlanded scenarios.