There are many resources (EPRI, boiler manufacturers, etc.) out there showing boiler chemistry limit recommendations for power boilers, however most of them address higher pressure units (usually > 1000 psi).

I have only seen one reference that gives more explicit guidance for phosphate chemistry limits below that level (B&W documents). Are there any other published guidance documents for low pressure boiler chemistry limits?

have you looked up NSTM documents? There are some info regarding low pressure boiler chem limits.
- Boiler Water/Feedwater Test and Treatment NSTM 220 Vol. 2 (Rev. 6)
- Boilers NSTM Chapter 221


BOILER WATER CONTROL LIMITS
CONSTITUENT or PROPERTY
REQUIREMENTS
ALKALINITY
0.100-1.000 epm
PHOSPHATE
10-40 ppm
CONDUCTIVITY
500 mmho/cm
CHLORIDE
1.0 epm

Do you inject Chelant into the water treatment?