After a low-key first half, Eberechi Eze came off the bench to head Palace into the lead before Janelt denied the Eagles their first league win of 2023 in stoppage time.
An early chance fell the way of Brentford in the seventh minute, but Christian Norgaard sent a volley just wide, and David Raya was required to deny Michael Olise's 18-yard drive just four minutes later.
Palace experienced fleeting moments of joy from set-pieces, with Joachim Andersen rising highest to send a couple of headers towards goal, but he could not generate the power to trouble Raya.
The Gtech Community Stadium crowd did not have a lot to write home about during the half-time break, with both sets of defences cancelling their opponents' out with relative ease.
Andersen and Ivan Toney were engaged in a well-fought battle throughout the contest, but the second half started in a similar vein to the first, with neither goalkeeper given much to think about.
As was the case in the first half, one of the few early half-chances of the second also fell to Norgaard on the half-volley in the 60th minute, but Vicente Guaita saved comfortably.
Arguably against the run of play, the opening goal would arrive for the visitors in the 67th minute, as substitute Eze connected with Olise's cross to head home.
However, in the sixth minute of second-half stoppage time, Janelt displayed his own aerial ability to connect with Bryan Mbeumo's cross and rescue a point for Brentford, who extended their unbeaten run in the Premier League to an impressive 11 matches.
Brentford remain eighth in the rankings and are next in action at home to Fulham on March 6, while the 12th-placed Eagles host Liverpool next Saturday evening.