If I can be honest, I don't know in short. Though it was something that seems very practical and it really frustrated me I couldn't figure it out, there must be an add-on somewhere for it,
Though I think I may have come up with an alternative solution. If I can make some assumptions, it is important that you manipulate a curve not a mesh....You are not using it for a path and only for geometry... and you will be converting to a mesh at some point...
If this is the case, this file may have a solution that is not correct but may work, I have split the curve into two separate curves with two 'vector' aligned 'handle' at the points of overlap. Also manually added a curve to create the position required for a bevel.
So now all you have to do, is convert both curves to a mesh (You can join the curves if you prefer), Join the two objects together with Ctrl + J and then remove doubles, and now you will have a line of polygons at the exact point you want a bevel, just select them and raise appropriately.
https://www.mediafire.com/folder/hhdz4v4fnhbwh/Assisted_Files
(make sure you keep the vector aligned handles exactly overlapped, keep the resolutions exactly the same and do not edit the 'bevel curve' without altering both, I would delete the internal curve on one of the objects if you want to edit it's position, re-seperate it and then attach it exactly to the other curve object or add hooks/handles to edit both curves simultaneously (would you like me to do this for you?))
I hope this worked for what you intend to achieve, unfortunately unless I know the exact reason you want this feature I cannot give you a more suitable way to achieve it, this was just my best solution so far. I do want to investigate more, this does seem to useful.
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