I see what you mean. I can't find a toggle to provide pre-insert on a Send.
I can offer that you go about your solution in a different way.
Use the powerful "Routing" capability instead of using the Send.
When you open one of the currently-inserted effects, you can click on the little tree (a node with two nodes under it).
Create two pathways: 1. your normal plug-ins 2. the plug-ins you wanted to use as pre-plug-in send.
Don't place any plug-ins on the original track.
Instead, send it to two different buses and do your parallel processing on those.
Hope this helps,