The technique for customizing your solution to allow editing of the Opportunity Owner (i.e. Opportunity.SecCodeId) value will vary based on LAN Client vs WebClient. There are also other caveats and customizations to consider.
All Opportunity related Activity, History, and Attachment records will still appear at the Account level even if you use the Opportunity.SecCodeId to secure the Opportunities themselves. In order to hide from Account level, would also need to add SecCodeID field to Activity, History, and Attachment tables/entities. If you do this, then need to consider writing logic to keep these Opportunity linked records SecCodeID value synchronized with their Opportunity's SecCodeID in case you allow editing of Opportunity.SeccodeId. Also need to write code to keep Activity/History/Attachment SecCodeId value synchronized with Account.SecCodeId for those records *not* linked to an Opportunity.
Finally - need to write logic to prevent edits to the Account.SecCodeId from over-writing any Child OpportunitySecCodeId values where the Opportunity.SecCodeId was not equal to the previous Account.SecCodeId value. The stock logic in the LAN and WEB products will automatically cascade (and overwrite) your Opportunity.SecCodeId values when an Account.SecCodeId is modified.
Todd Hardin
Sage Professional Services Group
