On the contrary, I feel this is very much possible, if my understanding is correct. I just replicated it using the steps below -
1) Made a simple report with a datetime report parameter. Made one text box and the value is set as the expression given below
=iif(instr(Parameters!Report_Date.Value,"19")>0,"http://bing.com","")
2) The same expression is given to the Jump to URL property in the navigation tab of textbox
3) Now preview the report. When you select a date like 19/10/2010, and then click on the textbox, the site bing.com will open up. But if a date which doesnt contain 19 is selected, then basically you will not be able to have the click option.
The expression can be substituted for whatever you want, and the url can be substituted for the report url
By the way, great session for the London SQL User Group meeting, found it pretty nice listening to you and Chris speaking on the new features of Denali. Thanks! And yeah, the rounded edges in SSIS look sexy! ;)