Hi Matt,
This is the method I last used and documented for me to re use.

copy Volvo DVD using nero 6.6.0.16 with the following settings
Target drive Nero 'image recorder'
Copy options - TICK On the fly (this may be the default)
Read options - Data DVD
Reading with error corection - retry count 1
TICK Ignore Read Errors

Applicable to LiteON Drives - check your make for similar utils.
Only if setting booktype to DVD-ROM
Open LiteON Booktype135.exe
change the setting so the next write is to be booktype DVD-ROM
LEAVE THE PROGRAM RUNNING

Burn with DVDDecrypter V3.5.40

Tools|Settings|Iso Write Mode
Mode AUTO
Type SAO
Link Size AUTO
TICK Enable Burn Proof, Lock Volume, Cycle Tray before Verify(Optional)
All others UN TICK
Layer Break for DL Media - Select Calculate Optimal.

Write Speed MAX

Start Write

After write is complete the LiteOn booktype util should report the disk as DVD-ROM


You are correct that only DVD+R can be set to DVD-ROM - For my Volvo reader disk type is not currently an issue it has read a disk of each type i.e. DVD+R DL booktype DVD+R, DVD+R DL Booktype DVD-ROM and DVD-R DL Booktype DVD-R.
The cut off point depends on how much are you prepared to spend making coasters.

Disks that worked
Traxdata DVD+R DL 2.4X (Ritek Disk)
Infiniti Professional DVD+R DL (not sure of speed think its 2.4X)
Verbatim DVD-R DL 4X

Hope this helps!
Good Luck
David