[WPF, .NET 4, VS2012, R#7] Custom markup extension and IntelliSense

We use a number of custom MarkupExtentension which act as shortcuts and most
of them merely set predefined converters, like in this example:

Visibility=""

The problem with this is the fact that the IntelliSense of R# does not work
in this case. R# does not recognize SomeBool. If I rewrite this using a
standard binding and a converter:

Visibility="{Binding SomeBool, Converter=...}"

R# will happily perform code completion on SomeBool using the current
DataContext.

Our markup extensions are derived from MarkupExtension, but even deriving
them from the Binding class makes no difference. Is support for
MarkupExtensions limited to the standard MarkupExtensions? How can we get
IntelliSense on our own MarkupExtensions?

Kind regards,
Henning