I eventually got the answer from Cisco engineering but since I couldn't find this documented anywhere on the Internet I thought it might be worth preserving it here for prosperity!
Theory
Unlike 1/10G operation where the device just detects what optic is inserted then presents the appropriate GigabitEthernet or TenGigabitEthernet interface, 1/10/25G ports need to be hard set into either 1/10G or 25G mode. Frustratingly, when it comes to 25 Gbps ports, the NCS platforms set this in groups of four ports, each of which is referred to as a"quad". In another confusing move, Cisco has decided to put the quad config in a completely different place to very similar things such as the config to break out 40/100G ports into 10/25G members.
Configuration
As we can see here, the 1/10/25G ports default to 25G mode, reflected in their TFx/x/x/x naming:
RP/0/RP0/CPU0:55A2(config)#do show interfaces description | begin 0/0/0/24
TF0/0/0/24 admin-down admin-down
TF0/0/0/25 admin-down admin-down
TF0/0/0/26 admin-down admin-down
TF0/0/0/27 admin-down admin-down
TF0/0/0/28 admin-down admin-down
TF0/0/0/29 admin-down admin-down
TF0/0/0/30 admin-down admin-down
TF0/0/0/31 admin-down admin-down
TF0/0/0/32 admin-down admin-down
TF0/0/0/33 admin-down admin-down
TF0/0/0/34 admin-down admin-down
TF0/0/0/35 admin-down admin-down
TF0/0/0/36 admin-down admin-down
TF0/0/0/37 admin-down admin-down
TF0/0/0/38 admin-down admin-down
TF0/0/0/39 admin-down admin-down
Mg0/RP0/CPU0/0 admin-down admin-down
Now, let's configure the first four 25G ports into 1/10G mode. The configuration lives under the "hw-module" branch, and quads are numbered from 0 (up to 3 on this platform). To set the first four ports into 1/10G mode:
RP/0/RP0/CPU0:55A2(config)#hw-module quad 0 location 0/0/CPU0 mode 10g
RP/0/RP0/CPU0:55A2(config)#commit
As we can see, ports 24-27 have transformed into 1/10G ports and now appear with the Tex/x/x/x naming convention:
RP/0/RP0/CPU0:55A2(config)#do show interfaces description | begin 0/0/0/24
Te0/0/0/24 admin-down admin-down
Te0/0/0/25 admin-down admin-down
Te0/0/0/26 admin-down admin-down
Te0/0/0/27 admin-down admin-down
TF0/0/0/28 admin-down admin-down
TF0/0/0/29 admin-down admin-down
TF0/0/0/30 admin-down admin-down
TF0/0/0/31 admin-down admin-down
TF0/0/0/32 admin-down admin-down
TF0/0/0/33 admin-down admin-down
TF0/0/0/34 admin-down admin-down
TF0/0/0/35 admin-down admin-down
TF0/0/0/36 admin-down admin-down
TF0/0/0/37 admin-down admin-down
TF0/0/0/38 admin-down admin-down
TF0/0/0/39 admin-down admin-down
Mg0/RP0/CPU0/0 admin-down admin-down
Let's set the rest to 10G mode, because 25G optics are expensive and I don't have any in the lab:
RP/0/RP0/CPU0:55A2(config)#hw-module quad 1 location 0/0/CPU0 mode 10g
RP/0/RP0/CPU0:55A2(config)#hw-module quad 2 location 0/0/CPU0 mode 10g
RP/0/RP0/CPU0:55A2(config)#hw-module quad 3 location 0/0/CPU0 mode 10g
RP/0/RP0/CPU0:55A2(config)#commit
RP/0/RP0/CPU0:55A2(config)#do show interfaces description | begin 0/0/0/24
Te0/0/0/24 admin-down admin-down
Te0/0/0/25 admin-down admin-down
Te0/0/0/26 admin-down admin-down
Te0/0/0/27 admin-down admin-down
Te0/0/0/28 admin-down admin-down
Te0/0/0/29 admin-down admin-down
Te0/0/0/30 admin-down admin-down
Te0/0/0/31 admin-down admin-down
Te0/0/0/32 admin-down admin-down
Te0/0/0/33 admin-down admin-down
Te0/0/0/34 admin-down admin-down
Te0/0/0/35 admin-down admin-down
Te0/0/0/36 admin-down admin-down
Te0/0/0/37 admin-down admin-down
Te0/0/0/38 admin-down admin-down
Te0/0/0/39 admin-down admin-down
Mg0/RP0/CPU0/0 admin-down admin-down
Easy (when you know how)!
