This is a strange requirement. A form is a design element that is shared by all users. You do not normally allow end users to change design elements, because the changes that they make will affect other users. Also, to make design changes directly, the end users must have Designer access. So this is usually a bad way to design an application.
It is not clear whether you really mean "All Releases" and "All Platforms". If you do, then this is the wrong forum. You need an R4 solution, because that will also work in R5 and Notes 6.
For an R4 solution, use the C API to insert the subform reference (which is a kind of hotspot). For most platforms you can do this in LotusScript, though it is not easy to code. You can find Windows samples in the Notes/Domino 4 and 5 forum that might be some help. You will need different versions of the code for different platforms.
For R5 on Windows you can probably use the Lotus XML Toolkit to insert a subformref element. This is relatively easy to code in LotusScript, and even easier in Java.
For Notes 6 you can use native LotusScript to insert a DXL subformref element. This is easy to code, and there are samples of similar code in this forum.