Haha, no I actually meant the Plangent Processes - there’s no indication of price or whether the hardware and software is actually for sale on it’s own, kinda has an aura of hyper-exclusivity about it on the website (Neil Young, Bruce Springsteen, Grateful Dead have used it).
Actually, digging deeper, it sounds like it may just be a service where one sends tapes or high res digital tape captures to them. Seems they do a similar thing to what I suggested with using Capstan on an isolated bias tone using their own “Clarity” software. Here’s a description from the liner notes of a Grateful Dead release:
The first step in this process was to transfer the original analog master tapes to a digital format. This was done by using a Pacific Microsonics HDCD A/D converter at the highest possible digital resolution, 192KHz/24bit (for comparison, a standard CD is 44.1KHz/16bit).
The resultant audio files were then sent via hard drive to Plangent Processes in Nantucket, MA. Plangent Processes utilizes a novel proprietary digital signal processing system called Clarity. > The process analyzes the tape, searching for artifacts of the recording stream that happen to contain highly accurate timing information, which then reveal the mechanical imperfections of the original tape recorder’s performance as it was making the initial master recording. To obtain this timing information, the original transfers to digital actually had to be done with the master tapes running at half speed, 3.75 inches per second.
This information was then employed as a guide to gently but firmly correct the data of the digital transfer such that the sonic performance in terms of speed, pitch and tempo was indistinguishable from the board output. In this case the original 7.5 inches per second recordings contained significant amounts of wow and flutter, as would be routine even in the professional machine used to make these recordings. By undoing the wow and flutter we can now hear the performances in their original perfect pitch, and with steadiness of rhythm and overall clarity intact.​
Playing the tape at half speed is a great idea that hadn’t occured to me, that means I’ll be able capture bias tones up to 192khz on my cassettes that were recorded at 9.5 cm/second by playing them back at 4.75 cm/second 
So my plan going forwards - capture the tape playing at half speed into adc at 192 khz, Then inspect with spectral editor to find bias tones. Then filter out everything else, run through Capstan, then apply Capstan correction to unfiltered files. Then resample to get back to normal playback speed. I’m keen to see if this actually works, but my 4-track is in bits on my workbench awaiting a chance to install a brand new playback head