Comment 1 for bug 518149

Using key modifiers on toolbar buttons would be a big change in UI behaviour that I do not know in other programs. As said in the bug description, split button might be an option.
However, in upcoming Inkscape 0.49, the user can set keyboard shortcuts for many frequently used commands (Edit > Preferences > Interface). For the proposal: the default shortcuts are:
*Paste in Place: ALT+Ctrl+V
*Paste Style: Shift+Ctrl+V

@Will: is this sufficient and does it conform to your needs?