When you open it with the Phoenix tool it should become decompiled. All modules will be in the dump folder. Search all for the DSDT string. Then you have start of the binary of the table. You have to cut it with an hexeditor and to save as a new file (dsdt.aml). Then you can transfer it to code with the compiler (asl /u dsdt.aml), find those values, change them and re-compile it again (asl dsdt.asl) Finally replace the DSDT at the original module and repack the BIOS. Which version do you want to modify? Please link to it. I can have a look 1A , 1B?
Oldest F.14 is also encrypted. Not sure how they managed to get it to work. It can be modded since andyP's tool can decrypt and encrypt it again, but not sure if the fact that it is encrypted will cause issues later when running a modified DSDT, are there any experiences?
make the HP_TOOLS partition (with the diagnostics UEFI tool) and keep the .bin and .sig of the modded bios in Previous and Current folders for occasional recovery needed.