Is it possible to create a custom NewForm for an external content type without using InfoPath?
I think so (although I never done it on external content type just on 'regular' lists).
You can create new external list (via UI or via Sharepoint Designer). Then you can use Sharepoint Designer to modify your NewForm.aspx (pick your list and click NewForm under Forms tab). Before doing some editing consult Google and look for some examples (mainly 2007 version).
There are also several other ways to edit content type forms directly. Here is one that I use (well it is pretty close): Walkthrough: Create custom SharePoint 2010 list form for deployment in a Visual Studio 2010 project
However, keep in mind I never used this technique on external content types so maybe there are few 'gotchas'