I tried looking up the order of events for a subreport but could not find when the subreport loads in relation to the main report. If it were a form, the subform loads before the main form. If the same thing happens with reports (the subreport "loads"/opens first) you might be able to insert your parameter value at that point. Have you tried stuffing your parameter from the dialog form via a variable to the subreport by using the Open Event of the subreport(report property page?)
Here is what HELP says for Order of Events for REports: (to maintain the contents I just pasted the whole thing)
Order of events for reports and report sections:
Events occur for reports and report sections when you open a report to print or preview it, or close a report.
Events for reports
When you open a report to print or preview it and then later close the report or move to another window in Microsoft Access, the following sequence of events occurs for the report:
Open Ã Activate Ã Close Ã Deactivate
When you switch between two open reports, the Deactivate event occurs for the first report, and the Activate event occurs for the second report:
Deactivate (report1) Ã Activate (report2)
The Deactivate event for a report also occurs when you switch from the report to another window in Access. However, the Deactivate event doesn't occur when you switch to a dialog box, to a form whose PopUp property is set to Yes, or to a window in another application.
When you open a report that is based on a query, Access triggers the Open event before running the underlying query. As a result, you can set the criteria for the report by using a macro or event procedure that responds to the Open event. For example, the macro or event procedure could open a custom dialog box in which you enter the report criteria.
Events for report sections
When you print or preview a report, the Format and Print events occur for the report sections after the Open and Activate events for the report and before the report's Close or Deactivate events:
Open (report) Ã Activate (report) Ã Format (report section) Ã Print (report section) Ã Close (report) Ã Deactivate (report)
In addition, the following events can occur during or after formatting but before the Print event:
Â· The Retreat event occurs when Access returns to a previous section during report formatting.
Â· The NoData event occurs if there are no records displayed by the report.
Â· The Page event occurs after formatting but before printing. You can use this event to customize the appearance of the printed report.