Figured it out:
When you click Merge Contacts in regular salesforce, you get: /merge/conmergewizard.jsp?retURL=%2F001E000001EAK2s%3Fnooverride%3D1%26sfdc.override%3D1&id=001E000001EAK2s
If you paste this in a Skuid button Redirect to URL, and take off the last id param values and replace with an Account ID, you’ll be in business:
/merge/conmergewizard.jsp?retURL=%2F{{{AccountIdField}}}%3Fnooverride%3D1%26sfdc.override%3D1&id={{{AccountIdField}}}
