How can I record a pressure map?
Pressure sensors arranged in a matrix to produce pressure related voltages which are scanned, digitised and analysed. Some pages on the site below say "pizeoelectric" pressure sensing and some say "resistive". Both are feasible. 


A compressible material which varies in resistance between top and bottom surfaces could be arranged in a matrix with each point scanned and measured.
Think in terms of semiconductor strain gauges, although not necessarily arranged in the usual bridge manner. 
A piezo element will produce a pressure related static voltage which can be scanned and measured.




SPI Sensor productrs Inc make a wide range of surface pressure indicating sensors. 



For a typical foot pad suitable for two feet at once, one system, The GoTec foot mapping sensor has the following specification:


Technology   Resistive
Number of Sensors   2,304 (Arranged in a 48 x 48 Matrix)
Spatial Resolution  0.31 in. (8 mm)
Sensing Area    15 in. x 15 in. (38 cm x 38 cm)
Data Resolution 12-bit
Pressure Range  0.72 to 30 PSI (0.05 to 2.10 kg/cm²)
Data Acquisition Frequency  180 Hz
Accuracy    ±10%
Platform Height 0.25 in. (0.63 cm)
Weight  9.25 lbs. (4.2 kg)
Operating System    Windows® XP / 7
Connection Method   USB / Wireless
Operating Temp. Range   5°F to 86°F (-15°C to 30°C)






The products shown below closely match your description.
Whether Scholls use the same method is unknown but it seems likely.

Their systems are based in some and maybe in all cases on piezoelectric pressure sensing elements (although "resistive" is mentioned) which are arranged in a matrix and then scanned and digitized by standard techniques.

Their Tactilus equestrics saddle fitting sensor is said to use piezo pressure measurement 


The Tactilus Equestrics® sensor is a matrix-based tactile surface sensor that works by the principle of piezoresistance. Tiny sensing cells cover the entire surface area of the sensor “skin” allowing for discrete spot pressure analysis at any point in the contact region. The Tactilus Equestrics® sensor provides real-time data showing precisely where the pressure points occurs between the saddle and the horse.




Specification: Note the use of the term "Piezoresistive" which places some doubt on whether the resistive and pizeoxxx systems are in fact the same.

Technology  Piezoresistive
Pressure Range  0 - 5 PSI (0 - 0.35 kg/cm²)
Grid Size   18 x 22 split mat
Sensing Points  388
Total Sensing Area  29.5 x 17.5 in. (75.9 x 44.5 cm)
Scan Speed  Up to 10 hertz
Thickness   30 mils (0.76 mm)
Accuracy    ±10%

"sensor products inc" advertise a range of tactile pressure indicating films.

Foot plate sensor



Sew-in piezoeresistive sensing material.
Unfortunately the page related to this image was dead :-(

Eeontex conductive fabric used for pressure sensor construction - data sheets here





Maxim - demystifying PR sensors](http://www.maxim-ic.com/app-notes/index.mvp/id/871) 


Abstract: Monocrystalline silicon pressure sensors have come into wide use in recent years. Though manufactured with semiconductor technology, they also operate on the resistive principle. The resistance change in a monocrystalline semiconductor (a piezoelectric effect) is substantially higher than that in standard strain gauges, whose resistance changes with geometrical changes in the structure. Conductivity in a doped semiconductor is influenced by a change (compression or stretching of the crystal grid) that can be produced by an extremely small mechanical deformation. Using a signal conditioning integrated circuit to temperature compensate and amplify the signal offers superior performance over discrete circuits. 


PE &amp; PR sensors U.Dayton.

PE &amp; PR Effects - COMSOL demo -  Flash so can't copy live image. Worth a peek.

Silicon PR sensors  useful.

PR design and optimisation somewhat off topic but useful and good references.

Northwestern edu - piezoresistive sensors 36 pages. Textbook chapter. Good. 

Wikipedia - piezoresistive effect 



"Body mapping":