Never mind. To externally reference const (which are implicit static), the declaration needs to be as follows:
-prog_char op_max[] PROGMEM = {
+extern prog_char op_max[] PROGMEM = {
In trying to figure this out, I had left in server.cpp
extern const char *op_max;
instead of
extern const char op_max[];
which seems to be deferenced differently and messed up the maximum values.