are there spl meters that are continuously variable ... fq wise, so basically i could play any fq between 20-20k and it would give me an accurate reading
Not sure if this will answer your question or not but -
almost any SPL meter will be continously variable in terms of frequency, but will not be accurate at the both ends of the spectrum. They mostly go upto 8khz or 10khz, and even then you have to factor in the meter's curve..
If you were asking about continously variable in terms of db level then your best bet is to adjust volume or position of the meter in such a way that your max db in the frequency range you want to measure is just below the max db of the db scale set on the meter. That way you have about 10db of headroom before you need to switch it to the next range. Yes it would have to be by trial-and-error.