We have MX BNGs doing PPPoE with PAP authentication method. Our RADIUS does not use the RGW/PAP username and password, but ADSL-Agent-Remote-Id for authentication, however our subscriber RGWs and BNG still have PAP configured, and the RGW may present a username and password in PAP. In production, some RGWs will send a blank username and password - i.e. they negotiate PAP as the authentication method, and then send empty string i.e. "" as the username and password - as the default RGW configuration is an empty string.

Juniper MX does not support empty username/password, with this log in authd traceoptions.

May 14 15:38:33.677211 Framework: Message failed sanity test - the username info is invalid

Does Juniper have a way to enable PAP negotiation, but override the username (and possibly password), rather than use the PAP negotiated username?

MX BNG will reject the PPPoE subscribers with empty username and password.

Starting from JUNOS 19.1, customer can use a hidden knob to override the empty username for PPPoE subscriber, thus make both PAP and CHAP work.

set protocols ppp-service local-username-override-when-empty <the_replaced_username_to_be_used>

For example:
<< set protocols ppp-service local-username-override-when-empty no-username >>

{master}

adminusr@BNG-re0> show subscribers user-name no-username 

May 14 09:49:59

Interface                      IP Address/VLAN ID                     User Name                     LS:RI

pp0.3221898456                 10.83.189.8                            no-username               default:default      

pp0.3222230998                 10.16.143.121                          no-username               default:default   

Junos can also override the password by using the override-password knob under [edit access domain map domain-map-name].

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/override-password.html

At the time, this knob is still in hidden mode. PR1810890 has been filed to unhide it.