I didn't see anything native in ArcPy. The easiest thing to do is probably send the F9 keystroke to the ArcMap window using the SendKeys module.
I tested with this syntax and it worked fine:
import SendKeys
# Send the keystroke for F9
SendKeys.SendKeys("{F9}")
The only caveat is that you may need to uncheck "Always run in foreground" on the General tab of the script properties. Otherwise the keystroke may get caught by the script progress window.
I had a similar issue to this and resolved it by deleting the corresponding cache directory for my add-in:
%LOCALAPPDATA%\ESRI\Desktop10.<x>\AssemblyCache where x is 1,2,3,4, or 5.
C:\Documents and Settings\<username>\Local Settings\Application Data\ESRI\Desktop10.0\AssemblyCache\
10.1 on Windows7:
%LOCALAPPDATA%\ESRI\Desktop10.1\AssemblyCache will take you to:
C:\Users\<UserName>\AppData\Local\ESRI\Desktop10.1\AssemblyCache
I also have in my notes to check this directory, but I don't recall if that was for while I was debugging in Visual Studio or just normal troubleshooting. :
c:\Documents and Settings\<username>\My Documents\ArcGIS\AddIns\Desktop10.0
If the AddIn that you are debugging exists here, try deleting it here.
Update
I knocked one of my add-ins and debugged. It does create a directory in My Documents\ArcGIS\AddIns\Desktop10.0 for debugging, which is unpacked to the AssemblyCache directory.
Best Answer
According to the documentation this can be done in the OnUpdate method, Add-in coding patterns