When I use the code given, it compiles and displays fine. But when I try replacing \everypar with \EveryPar, it halts during compiling with this complaint:
"A number should have been here; I inserted '0'. (If you can't figure out why I
needed to see a number, look up 'weird error' in the index to The TeXbook.)
mtx-context     | fatal error: return code: 1

"

    On Monday, October 10, 2022 at 09:58:11 PM MDT, Wolfgang Schuster <wolfgang.schuster.lists@gmail.com> wrote:  
 
 Max Chernoff via ntg-context schrieb am 11.10.2022 um 00:57:
> Hi Joel,
>
> On Mon, 2022-10-10 at 12:46 +0000, Joel wrote:
>>  Hello Max,
>> It is preferred if the solution is just three lines per paragraph,
>> rather than some content parallel to the text
> A Lua callback solution:
>
> [...]
>
> An \everypar solution:
>
>    \appendtoks%
>        \vbox to 0pt{%
>            \dorecurse{3}{%
>                \rlap{%
>                    \hskip\dimexpr\hsize+1em%
>                    \vrule height 0.4pt width 3cm%
>                    \relax%
>                }%
>            }%
>        }%
>    \to\everypar

You can use the \EveryPar register but \everypar is off limits.

Wolfgang