Front End entry form, mapping a hidden field to a plugin?
It's probably too late to help the OP (sorry @shorn), but here's how you would pull this off with Smart Map...

Add the Address field on the front-end.

&lt;label&gt;Street Address&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][street1]" value=""&gt;

&lt;label&gt;Apartment or Suite&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][street2]" value=""&gt;

&lt;label&gt;City&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][city]" value=""&gt;

&lt;label&gt;State&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][state]" value=""&gt;

&lt;label&gt;Zip Code&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][zip]" value=""&gt;

&lt;label&gt;Country&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][country]" value=""&gt;

&lt;label&gt;Latitude&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][lat]" value=""&gt;

&lt;label&gt;Longitude&lt;/label&gt;
&lt;input type="text" name="fields[myAddressField][lng]" value=""&gt;


Then, you can inject the lat and lng values using JavaScript. Since you're doing that dynamically, you could even make those inputs hidden.

That's it!