Fill auxillary channel with value

What's a good method for filling an entire auxillary channel (Z depth) with a given value?

In my case I want to take two things that don't have any Z-depth information, add Z-depth to them , and use that for a Depth Merge. This would allow me to control the layering of these elements dynamically.

Thanks!

ChannelBooleans is what you want. Leave it in Copy mode, set RGBA to "Do Nothing" then go to the Aux Channels tab, tick "Enable Extra Channels" and choose the channel you want to copy to Z in the "To Z Buffer" combo box. Connect the image you want to insert the new channel into to the Background and the image providing the new channel to the Foreground.

You will probably find, though, that your edges don't look very nice with this method because the alphas won't be processed the same way as in a 'proper' Merge. In order to get decent results from the Depth Merge, you'll probably also need to build a Z Coverage channel, which tells Fusion which pixels are meant to be anti-aliased. I'm not really sure how the coverage channel works, though; I've never needed it.