Will I be able to use RGsB for 480p games through the OSSC (either passthrough or x2 for 960p) and feed that output to the Framemeister which then uses HDMI passthrough without doing anything to the signal and without adding any additional lag?
For 480i games I’d like to use passthrough on the OSSC and let the Framemeister do the upscaling.
Does this setup make sense and will it work like that without adding any noticeable input lag to the chain?
To start with, since you already have the OSSC, I suggest:
* Setting 480i/576i proc to passthrough to let your TV deinterlace; and
* enabling automatic input switching, so that the OSSC will automatically pick up the switches between RGBS and RGsB.
If your TV accepts 480i over HDMI, then that setup will let you continue to use RGB from the PS2 and get deinterlacing of 480i without having to buy anything. You can also try setting 480p/576p proc to line2x (960p), and/or 480i/576i proc to line3x (1440i), but your TV may not like those modes.
If, however, your TV does not like 480i (neither passthrough nor line3x) over HDMI, then, yeah, you’ll need a video processor in between. Most scalers that would help you are going to add a small amount of lag (at least 1 frame) and will suffer from HDMI blackouts during mode changes.
Personally, for a time*, I’ve used an Extron DSC 301 HD with my PS2 (Well, really everything running through my OSSC), which has worked well for me, and looks good to me, but other (and more knowledgeable) people have said that its deinterlacing is not good, and, if you have the OSSC chained into its HDMI input, you can expect HDMI blackouts to take about 3 full, agonizing seconds (versus about 1 full second using the VGA input via an active HDMI to VGA converter).
(*I say for a time, because my processing chain is currently in disarray and needs to be redone.)