Side voicing comes to mind if you don't want to use chemicals. It probably won't do much but it may soften them just enough to allow regular needling. I've never had much success with side voicing and don't care for it much, but if you're desperate….
Where are you inserting from and where are you aiming for?
Try inserting at the strike point aimed at 2:00 or 3:00 and similarly toward 10:00 and 9:00 so that you're only penetrating perhaps 1/8" below the surface of the hammer and effectively loosening just the outer layers. Then back up a bit so you are inserting at 11:30 and aiming at 1:30 which is a cross stitching move. This way you aren't trying to penetrate too deeply into the hammer and you are slowly loosening the outer layers.
The other more radical method is called compass point voicing and involves using a literal compass point held in a single needle pin vise. You aggressively needle straight down on the strike point (stabbing motion, not pressing) and then set the felt with a small hammer after you're done. This method definitely works both in hammers that are too hard and hammers that are over lacquered. In send a picture if you want.
"After working on this idea for several years, I am comfortable and enthusiastic in recommending Ballistol spray as my voicing method of choice for any hammers that need softening. The result is full tone and range without taking away the forte.
"Hard hammers are found in most Asian pianos as well as older pianos and are frequently difficult to voice down. They all respond well to Ballistol. I have used it for voicing every kind of piano from a spinet to a Steinway B. The effect is marvelous, and my customers love the change. It softens hammers by adding back a synthetic oil to replace the lanolin that is often extracted in the making of hammers, or has hardened over time. It is naturally antibacterial and anti-fungal, so it will not promote mold growth. The added resilience of the treated hammers produces a bloom often not heard in Asian pianos. …
"Depending on the degree of voicing down needed, and the size of the hammer, I use two to five seconds of spray per hammer, applied at the strike point. This can be done in a grand through the strings without pulling the action. Using the supplied straw, I spray it for one to two seconds and then play the note to assess the results. I often follow it with another one to two seconds or more per hammer, at least in the midrange. Less treatment is needed in the highest octave. Often the bass does not need much, but a proportionately larger dose of Ballistol will add power and bloom to the bass. For the occasional hammer that stands out a little bright after treatment, I will give it an "angel shot" (Journal, December 2011 and January 2012.) I also find it useful to compact the hammers after treatment for added power. To do this, I block the string with my finger near the strike point and give the key five hard blows on each note.
"Ballistol voicing is very forgiving and nearly impossible to overdo. It can take as much as three to six ounces to treat all the hammers in a piano with very hard hammers. It only takes a few minutes to get a result that might take an hour or more with needling. I now voice nearly every piano at the time of tuning with a minimal charge for the service. Every pianist so far has loved the result.
"A Ballistol 6-oz. spray can is inexpensive and readily available online.
"Doug Gregg"Long-Island Suffolk NY Chapter"It's an appealing idea. Subsequent Journal discussion revealed that Ballistol is commonly used by European tuners for a wide variety of needs. Thoughts?