I think you can try this without a D.I. or some sort of impedance converter.
In Reaper, you can achieve this by sending the track you want to process to a free output on your interface.
As you don't have a free output on your 2i2 interface, you can temporarily change routings to use one output as a send channel to the effect pedal.
Use the routing window to disable the master send on the track you want to process, then add a mono hardware output to the chosen channel(let's say the right output). (disable direct monitoring on the interface!).
Start off with a low gain setting on the sending track, so you don't hit the input of your effect pedal too hard. (line out is a higher voltage than the output signal of an electric guitar. Lower volume -> lower voltage)
On a second, empty track for the return signal, choose the input you will use for the return signal. Set record monitoring to off and turn on 'record arm' for that channel.
You will want to monitor the sound returning from the effect pedal. For that, open the routing window on the master track, remove the stereo hardware output and add a new mono hardware output to the channel where you still have a speaker connected (left output). Then set record monitoring on the receiving channel to on and hit play. Set the gain levels on sending track, effect and input gain. Turn the dry/wet knob on the pedal all the way to 'wet', you can mix reverb and dry signal later in the DAW.
When satisfied with the sound, hit record to print the return signal from the reverb pedal onto the new return track.
There will probably be some latency on the recorded return signal, but that is easily solved by nudging the recorded sample a bit. (or not if you like some extra pre-delay)
Good luck! And let me know of anything is unclear