This may not be the most elegant solution but you can try using parameter files. Here is an example:

1. Create some data.
   Flow('data',None,'spike n1=100 | bandpass fhi=40')

2. Generate a parameter file from it.
   Flow('mag.par','data',"attr want=max | sed 's/value = /mag=/' ")

3. Use the parameter file in processing.
   Flow('test','mag.par','spike n1=100 par=$SOURCE',stdin=0)