If you want to know where to start, search GitHub for 'KaveriPI', if you unpack AMD BIOSDBG.EXE you can find a complete list of processor registers. This is all from 2015 but the PSP is documented in there.
There's also a Microsoft Access database which has all the JTAG registers, but I don't have the time to decode the meaning of the fields... It is likely that things have changed since then but it still might be enough for a start.
Should the JTAG interface be protected then some kind of laser(?) fault injection might be required to open it up. I guest some of the eFuse bits can be overwritten, maybe there's a combination which can remove the lock. An innovative recycling company could work on making a jig to automate this somehow...
Some PSP JTAG stuff here (publicly available material from GitHub in 2015, fair use applies): 41469,3529,164000,164999,"SMU_PSP_efuse_ovr_tried",,1,0,0,0,50,"0000",0, 41470,3529,164000,164999,"SMU_PSP_FRA_pass_ld_err",,1,1,1,0,50,"0001",0, 41471,3529,164000,164999,"SMU_PSP_FRA_pass_ld_cor",,1,2,2,0,50,"0002",0, 41472,3529,164000,164999,"SMU_PSP_efuse_pdmb_aes_dis",,1,3,3,0,50,"0003",0, 41473,3529,164000,164999,"SMU_PSP_efuse_pcpu_dis",,1,4,4,0,50,"0004",0, 41474,3529,164000,164999,"SMU_PSP_efuse_ccp_cyph_dis",,1,5,5,0,50,"0005",0, 41475,3529,164000,164999,"SMU_PSP_efuse_FRA_en",,1,6,6,0,50,"0006",0, 41477,3529,164000,164999,"SMU_PSP_efuse_proto",,1,7,7,0,50,"0007",0, 41478,3529,164000,164999,"SMU_PSP_efuse_secure",,1,8,8,0,50,"0008",0, 41552,2352,164000,164999,"SMU_PSP_hard_resetb",,1,31,31,0,50,"101F",0, 41553,2352,164000,164999,"SMU_PSP_early_resetb",,1,30,30,0,50,"101E",0, 41554,2352,164000,164999,"SMU_PSP_slv_mbus2_reset",,1,29,29,0,50,"101D",0, 41555,2352,164000,164999,"PSP_SCAN_MODE_STICKY",,1,28,28,0,50,"101C",0, 41568,2352,164000,164999,"PSP_AEB_307_PCPU_RST_DLY_TDR_en_pclk",,1,15,15,0,50,"100F",0, 41569,2352,164000,164999,"PSP_AEB_304_PCPU_FORCE_rst_en_pclk",,1,14,14,0,50,"100E",0, 41579,2352,164000,164999,"PSP_Resetn",,1,8,8,0,50,"1008",0, 43605,555,164000,164999,"PSP_ENABLE_SPARE",,0,1,1,0,50,"0001",0, 43642,555,164000,164999,"PSP_SPARE",,0,7,14,0,50,"0007",0,