Using ClientId in a server control's attribute declaratively

Hi there
I have a server control with a property (attribute) which I need to specify declaratively like this:
<blah:MyControl runat="server" MyProperty="something"/>
However, that attribute value needs to contain the ClientId of another control. I can't use this kind of syntax:
<blah:MyControl runat="server" MyProperty="<%=someothercontrol.ClientId %"/>
Because of the limitation of doing that within server control.
Is there ANY other way I can do this? I really don't want to have to declare the property programmatically.
Thanks
David

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