Interfacing with a HUD

In SL, anytime i've gotten a HUD I have to unpack it then go an extra step into my inventory to wear it. However when I purchase clothes for my avatar, it asks me immediately after purchase to wear it and therefore not have to go to my inventory to find it.

How can I accomplish this same immediate wear mechanic as I do with clothing as with a HUD?

You can't. (Yes, that's a terse response especially for me but really, what else needs to be said.)

It's a little kludgey, but if you're the one selling and scripting the attachment, it's possible to get a kind of similar behavior by calling llAttachToAvatar() when you get a CHANGED_OWNER event, if you can sell it Original (that is, a vendor object would have to rez a fresh copy after each purchase). Sample scriptlet:

default
{
    changed(integer change)
    {
        if (change & CHANGED_OWNER)
            llRequestPermissions(llGetOwner(), PERMISSION_ATTACH);
    }
    run_time_permissions(integer perms)
    {
        if (perms & PERMISSION_ATTACH)
            llAttachToAvatar(ATTACH_HUD_CENTER_2);
    }
}

To test such a thing, you'd need an accomplice (alt) to do the buying (else no CHANGED_OWNER event).

There's at least one weird glitch (http://jira.secondlife.com/browse/SVC-1765) with things that attach while rezzed in-world (as opposed to attaching from Inventory), but the one I know about affects particles, which are pretty unlikely to be part of a HUD.

There is the other glitch, where stuff attached from in world instead of inventory has a tendency to vanish. Save it for things that aren't valuable.

You're right, it's a bit kludgey! I'm waiting for the "BAMF* to happen next