I've realised it's quite useful that using the RBD Fractured Object DOP, and the "Create Convex Hull per Set of Connected Primitives" flag, Bullet will interpret Houdini (box, sphere, tube) primitives as native primitive colliders instead of converting them into convex hull meshes, and it can be used to easily bring in a whole bunch of efficient compound colliders straight from SOPs.

...but, Houdini doesn't have a "capsule" primitive type - I can get close enough just bringing in a cylinder and two spheres, but it'd be nice to have it just use one in place of the three.
Can anyone think of a way to use this method and bring capsule colliders into Bullet?
I've dug through the DOPs spreadsheet to see if I can fudge it using some Script Solver python, but as far as I can tell, Bullet does its own collider generation internally, so there's nothing there to change beyond the SOPs geometry being fed in.

...aaand in keeping with tradition, after hours of digging around, I discover the answer within five minutes of posting a question on here :-)

The answer is the "Capture Region" SOP, for anyone interested... apparently Houdini does have a native capsule representation - a tube with a 6xFloat "tubeCapt" primitive attribute attached.