In a Marriage entry you have this box which is where you can enter a minister:

- ScotMarr-1.PNG (942 Bytes) Viewed 138 times
On the Death entry you have this box box which is where you can enter a Registrar:

- ScotMarr-2.PNG (881 Bytes) Viewed 138 times
All I've really done there is just change the label from 'Minister' to 'Registrar'. So there isn't an equivalent additional box to also enter the marriage registrar because that box is already used to enter the Minister who officiated at the marriage.
A death registration date is generally quite important because the death is registered by a member of the family and all certificates from multiple countries use this information. This is why I added this to Ancestral Sources in the first place. A marriage registration date isn't something that appears on certificates generally (or isn't relevant as far as I can understand it - see comment below)
I think what you're asking for is not a change to a template but a change to the Ancestral Sources application so you can have similar additional boxes to type in the name of the registrar and the date. This would require significant additional programming from me and I don't really see much benefit from doing so. All you would be doing is typing the name of the registrar and the date into two boxes on the screen and that's what you're already doing when you type them into the boxes in the source text below.
An additional comment: I'm not really sure what the significance of a marriage registration date (and who registered) or why this is important? I'm not sure how this would affect my family history. I can see from the Scottish marriages that they tend to be registered a day or two after the marriage. Is this just an administrative job that takes place behind the scenes - is this something that the married couple get involved in at all? I take your point that it may be that the registrar is a member of your family but wouldn't that be something that would affect only a tiny fraction of a percentage of users of AS (probably 0%)?