Dents in steel bikes aren't usually a problem. Actually a "dent" can give form stability to a round tube. Take some of the Colonago steel framesets. I would say that you could safely ride your bike for some time before you have to worry. But the picture shows a significant "crease" dent orthogonal to the tube. A significant sideways impact would cause a problem. I think it will eventually crack due to corrosion. Apply some paint and ride it until you want to replace the tube. This is an older steel frame and is fairly thick tubing compared to some of the recent frames. A good thing is that it is a lugged frame and the top tube can be replaced easily. It would probably cost $200 to replace the TT and some $$ to repaint the TT and areas around the lugs. Check with a local builder of contact Nobilette (Longmont, CO) or Bilenky (Philly I think). They can do very good frame repairs.
Forget the steel ball idea. Frame alloys for steel bikes have a very high modulus and aren't easily repaired. The steel on autos is very soft comparably. There is a set of rollers that can be used to roll out some of the dent's flare but, there will be some paint damage and the crease will remain. If you have a good frame builder in your area take the bike by. This is the time of year that builders can get to repairs because the new bike orders are down.

I recently dented the down tube of my Columbus Neuron (lightest of all steel tubing) in a crash. It is on the bottom of the DT so I don't see it (out of sight out of mind). The bike is a Mondonico Diamond Extra moltini orange. I will eventually replace the down tube and put in S&S couplers in for traveling. I am going to wait until it needs a paint job. Right now it is still beautiful.
I am not worried about the dent and would use the bondo route if I wanted cosmetic perfection. The paint wasn't damaged. I just wax my frames periodically to prevent any problems (and it makes the frame look nice). The frame alignment wasn't compromised and there are no twitches in fast descents so the frame is "good."
I recently sold an old 1999 Columbus FOCO tubing frame. The Shaped tubing makes for a very tough frame. It was thin but my Neuron tubeset is even thinner (and round). Unless the bike alignment or performance is in doubt don't worry. The only frames I have seen crack is from front end crashes where the top and down tubes are both bent across the whole tube diameter and the paint was compromised. I bought an old Torelli cross frame built with "seamed" tubing that was in a front end crash. I rode the bike for 3 years before I noted a crack in the tubing. I will eventually replace the tubes of the lugged cross bike when I have some Idle cash (a joke these days). Another nice thing about steel is that it will crack slowly and usually not fail catastrophically like Aluminum, Ti, or Carbon from a stressed frame. Ti is a very strong material but I have seen failure around welds from embrittlement caused by Oxygen during welding.