Sorry gents if I'm talking about something different (Flash 7.0.19.0 is here) but some time ago we discussed how to include Windows ASPI driver into installation. Why not to try including Flash in that manner - through HFEXPERT folder

The only file you need is FLASH.OCX (I'm talking about Flash 7 but I think it should be nearly the same even if there are a few files needed for Flash 8).
Place the file into HFEXPERT\WIN\SYSTEM32.
Create FLASH.INF file and place it into HFSVPK:
CODE
[Version]
Signature=$CHICAGO$
[DefaultInstall]
AddReg=Filter.Reg
[Filter.Reg]
HKLM,"Software\Microsoft\Windows\CurrentVersion\RunOnce\Setup","Flash.ocx",,"%11%\regsvr32 /s %11%\flash.ocx"
You are done

You should only find out what files are really needed for Flash 8 and if FLASH.INF should be customized a bit according to the new version (maybe added some settings).
And if you want you can create a switchless installer with IEXPRESS.EXE (included in Windows