There are only 3 ways to descend below MSA.
1) Visually
2) Under Radar control
3) Following a full procedure.
Flying to a self positioned waypoint should be treated with the utmost caution.
In Sabenaboy's example self positioning from the south and descending to 3000ft within 25 miles would be OK as it keeps you above MSA. From any other direction this would be a no no as MSA is higher so follow the full procedure. You need to be within 30 degrees to commence an outbound and more importantly must be within 5 degrees to actually descend on it. Be very wary of ATC with no Radar. You're flying the aeroplane not them.