Its possible by modifying the ISV.config (XML). http://msdn.microsoft.com/en-us/library/cc150910.aspx
You'll want to look at adding an entry your entity under the following xpaths (account example shown)
/ImportExportXml/Entities/Entity[name='account']/Grid/MenuBar/ActionMenu
or
/ImportExportXml/Entities/Entity[name='account']/Grid/MenuBar/Button
Example in SDK is:
<!-- The main Global Menu Bar located at the top of all root level areas -->
<MenuBar>
<!-- Custom Menus that appear between the Goto Menu and the Help Menu -->
<CustomMenus>
<Menu>
<Titles>
<Title LCID="1033" Text="ISV" />
</Titles>
<MenuItem Url="http://www.microsoft.com" >
<Titles>
<Title LCID="1033" Text="New Window" />
</Titles>
</MenuItem>
</Menu>
</CustomMenus>
</MenuBar>
The MenuItem node supports a JavaScript attribute (make sure you XML encode your javascript).
Not sure what you want to parameterize to your url. If you use a modal dialog, the selected records in the grid are passed as dialogArguments on the window. Remember that IE has a maximum URL length if you were planning on passing many selected GUID's by query string. Full MSDN example: Walkthrough: Capturing the GUID Values of Records Selected in a Grid