Type 15 is an empty slot, so you can put your custom definition in there.
There is a risk with using type 15, because of the way in which type 15 ended up being used as a flag in late model IBM 5170s, and most 5170 clones.
FIRST IBM 5170 BIOS
In the CMOS/RTC chip, only one byte (at address 18 decimal) is used to store the drive types for both C: and D: In that byte, the upper nibble is used for drive C: and the lower used for drive D:
Address 18 (decimal): C_TYPE_NIBBLE/D_TYPE_NIBBLE
Each nibble (being a nibble) can store from 0 to 15, and that is why the first BIOS only goes as high as drive type 15. Type 15 is reserved.
SECOND/THIRD 5170 BIOS
Drive types greater than 15 are now supported. IBM did this by using the reserved type 15 is a particular way.
If the C: drive nibble at address 18 is set to 15, it means that the C: drive type number is greater than 15, and is stored in a byte at address 25.
If the D: drive nibble at address 18 is set to 15, it means that the D: drive type number is greater than 15, and is stored in a byte at address 26.
Address 25 (decimal): C_TYPE_BYTE
Address 26 (decimal): D_TYPE_BYTE
Example:
With drive C: being type 34, the C_TYPE_NIBBLE at address 18 is set to 15, and the C_TYPE_BYTE at address 25 is set to 34.