I was implementing the Business Data List Web Part on an External Content Type and realized I didn’t have any custom actions. The only way to create a custom action is to create it in Central Admin. My workaround for this is to add a link to a list form in an external list on the same site in the XSL of the Business Data Web Part, using the SharePoint JavaScript modal dialog.

<a onclick="OpenPopUpPage('https://myportal.com/sites/test4/Lists/CorporateTraining/DispForm.aspx?ID={@BdcIdentity}', null, null, null, null);" href="#">
<xsl:value-of select="@RecordId" /></a>