Given MEG is an approximation of neuronal spiking, are there any models that associate the spiking of simulated neurons to an MEG signal? Ideally, this would be a model with some amount of behaviour (mapping to an output, such as a motor action), but purely biological models are fine too.
Techniques that measure brain activity at a distance such as EEG and MEG measure spiking activity very poorly and indirectly. Instead, these techniques primarily measure synaptic currents.
Synaptic currents of course originate in some way from spiking activity, however:
Synaptic currents may not occur in the vicinity of the spiking cells
The amplitude of synaptic currents depends on the current state of the cells impinged upon
Synaptic currents of different types in the same area can cancel each other out
Phase, polarity, and geometry can lead to a complex inverse problem at the sensor that has an infinite number of solutions.
However, by making some rather strong assumptions, one can attempt to solve this inverse problem. Higher frequency EEG/MEG activity is more closely associated with spiking activity, and therefore most appropriate for attempting to localize spiking rather than other types of activity. See for example David and Friston 2003.
I'd also add that although the current agreement seems to be that EEG/MEG primarily come from synaptic currents, because synaptic currents do originate ultimately from some sort of spiking, EEG/MEG signals can correlate well with spiking activity measured with depth electrodes (for example Whittingstall & Logothetis, 2009). It's just likely that the correlation is indirect, mediated through synaptic potentials.
Buzsáki, G., Anastassiou, C. A., & Koch, C. (2012). The origin of extracellular fields and currents—EEG, ECoG, LFP and spikes. Nature reviews neuroscience, 13(6), 407.
David, O., & Friston, K. J. (2003). A neural mass model for MEG/EEG:: coupling and neuronal dynamics. NeuroImage, 20(3), 1743-1755.
Whittingstall, K., & Logothetis, N. K. (2009). Frequency-band coupling in surface EEG reflects spiking activity in monkey visual cortex. Neuron, 64(2), 281-289.