In case anyone ever ends up here looking for a way to achieve what I asked for in my original question (although it won't solve the other issues I wanted to use this for as described in the later posts): you can cheat by creating a character with the properties of the item you want to use.

1) open the templates (the list of all items, characters, triggers, ... you can create)
2) click on the third button at the pane's top (with the "+" on it) to create a new template
3) the default type will be "character", so that's already fine
4a) click on the field next to "PhysicsResourceID", then on the "..." at the end
4b) click on the second icon in the "pop-up field" to open the resource manager
4c) select the physics resource of the item you originally wanted to use (you can right click on the item in the template list and select "edit" to see this resource's name; in the resource manager, choose the item with correct name and the green icon)
4d) after clicking in the resource manager, the pop-up field" from step b) will have disappeared. Therefore, repeat steps 4a) and 4b) after selecting the physics resource in the resource manager, and this time click the first icon in the "pop-up field" (the left arrow) to fill in your selection
5) do the same as in 4) for the VisualResourceID (bleu teapot icon)
6) Set the AI bounds (the first field of the dialog box). Unfortunately, it appears characters are limited to cylinder bounds
7) uncheck "CanFight", "CanOpenDoors", "CanSeeThrough", "InfluenceTreasureLevel", "SpotSneakers", possibly also "CanBeTeleported" depending on the kind of item
8) change BloodSurfaceType to "None"
9) You can set the "Stats" to that of a character whose properties match those of your object (or I'm sure there are posts on this forum explaining how to create new character stats files), possibly also increasing the level to increase hitpoints etc.
10) change the HitFX to something that's not blood (e.g. maybe FX_GP_Hit_Metal_A_Small)
11) under "4) Select lsb" in the dialog window, select an lsb (you can add a new one by entering a name in the field next to the pop-up)
12) enter a name under "5) Name" (it will have been filled in with the name of the item whose resources you selected, but you can't use that name since it's already in use)
13) click on the "Create" button

I have only tested basic functionality of the resulting "character" (a CharacterMoveToTrigger() in the story editor), but I assume it can do anything a regular character can do as far as the Osiris and the charScripts are concerned (of course, lack of animations means you won't have any walking/running/being angry etc).

The main downside I've found until now is that due to its character nature, you'll always get a health bar when mousing over it (even if it's supposed to be an indestructible object like a bench).

Last edited by Tinkerer; 02/04/16 11:08 AM. Reason: minor correction to 4c), mention icon of visual resource ID in 5)