Spent some time looking at CIC filters and I think I understand them better now. Here is a good paper:

http://www.dspguru.net/sites/dspguru/files/cic.pdf

Equation 8 is the key, though it is clearer when explicitly expanded. CIC I/O response is:

1 + (z^-1) + (z^-2) + ... + (z^-(RM-1))

This is obviously a FIR filter with all coefficients =1, which is the definition of a boxcar or moving average filter with a rectangular window of width RM and a gain of RM. When decimating with a CIC we can place the comb downstream of the decimation, which is quite convenient because it reduces the required comb delay by a factor of 1/R. If M=1 and the comb is sampling every R samples, then what it is sampling is the last R input samples added together, which is the definition of an integrate and dump filter.

CIC makes sense in scenarios where the oversampling ratio is high and one needs to radically filter and decimate in order to get the sampling rate down to something more reasonable. A cascade of 3 seems to be the norm to get a good cutoff slope. Bit growth is an issue, particularly when cascading, though modulo math in the accumulators eases this. And the decimation ratio takes the place of lots of M storage, which is really convenient, but if you don't want to decimate then you have to pony up the storage.

Anyway, I'm back to first order integrate and dump with power of 2 periods. Though since interrupts shouldn't come in at more than a 1.5MHz rate I'm toying with the idea of putting an interrupt time stamp in the Hive register set, which could enable period timing with the interrupt mechanism alone, though it would pretty much tie up one thread. (But first I'm off in an attempt to rearrange the Hive guts for full 32 bit memory access.)

I'm still kind of concerned about aliasing. I don't believe I've really thought it out, particularly in the context of heterodyning. To reduce the probability of it, one could take the LC oscillator output and LPF the phase with a PLL or DPLL before heterodyning. Sampling and filter internal node truncation are huge cans of worms, analog filtering is a breeze compared to digital filtering. Even something as trivial as the CIC, which boils down to almost nothing, just about breaks my brain. (Though the CIC itself is a very clever arrangement.)