The Pi might not be powerful enough, but have you seen https://github.com/tenox7/wrp/

No, this is new to me, thank you. My question then, is what are the limitations of this proxy? Are there alternatives to it that do the same or more? I ask so I know more of what to look for and what my options may be.

OP, Is your retro system limited to something like a serial cable connection? (I'm asking because you used the phrase 'modem emulator'.) Could it possibly use an Ethernet connection to the Pi?

I was originally going to suggest something like a VNC client or perhaps something like a local X server on the retrobox/remote X client on the Pi configuration if you had enough bandwidth between the retro box and the Pi, but if you're limited to serial port speeds of less than 115.2K, that's not really feasible.

Re: the WRP proxy project linked to earlier in the thread.. If the OP's system is limited to serial cable speeds, loading most Web pages is going to involve the retro box sucking down an image that's the size of the entire current page... probably at least a meg of downloading through something the speed of a dialup modem. (and in this case, I'm hoping serial means 115.2K and not 9600 or 2400...)

Personally, I'd lose my patience and just suck it up and use Lynx or Links through a text terminal on the Pi. *shrug*

It depends, some of the machines are limited. In my collection is an Amiga 1000, Atari MEGA STe, MSX Turbo R, X68030, 286, 386, Pentium Overdrive, Commodore 128D, Mac 512, etc. to name a few.

Can any of those 8-bit systems run their own TCP/IP stacks and some sort of text based browser? If not, I'd think they'd be stuck as text terminals running copies of Lynx or Links on the Pi.

The 286 could probably run Windows 3.1 or a really recent version of Geos/Ensemble/Breadbox, using a SLIP connection to the Pi and either Breadbox's native browser or Mosaic/Netscape for Windows. One of those might be enough to use WRP to browse some sites, or maybe WebOne (listed here on this forum) if you were willing to forgo remote rendering to modern standards.

When you get to the 386+ level, things get a lot easier. Windows 9x and early NT OSes can run a wider range of early browsers that should be able to work with WRP, and can be configured with SLIP or PPP to link to your Pi.
Alternatively, Arachne under DOS might be workable for any of the early x86 systems, (it has its own SLIP and PPP drivers) but I don't remember if the later Arachne versions are capable of doing clickable image maps (which is necessary to navigate pages with WRP).

Also, I just found this on a recent Google search: http://arcady.chem.anrb.ru/vncdos/
The developer says this can be used on DOS systems going back to XTs (presumably with either CGA or Hercules cards). Although I don't think VNC is really a good idea at serial port speeds, it might work at really low colors.

When it comes to the Macs, I've run systems from the Plus up to 68040-era systems with MacTCP, System 7 and some early browsers. (Mosaic, MacWeb, MacLynx, early Netscapes and IE versions on the later Macs) I'm not sure the 512 can handle that, though.. same for the Amiga 1000 if it's stuck with Workbench 1.2. Those might be terminal-only systems too, unfortunately.

The X68K series machines and the Atari aren't really familiar to me, but a quick browse of Wikipedia shows that they're in the same CPU & RAM range as some of the Macs that I've used with graphical browsers before..anyone want to chime in?

Something I was thinking about while getting ready for work:
Most of the systems on the OP's last post, including the higher-end 8 bit systems (the Commodore 128 and the MSX TurboR) have graphical operating systems available (GEOS for the C64/128 and SymbOS for the MSX), and could probably handle limited hypertext browsing if you didn't have to stuff an entire HTML rendering engine into a space small enough to share with an operating system in 64-512k.
Perhaps a server system could load the full HTML of a modern web site, process & convert it to smaller chunks of a custom bytecode protocol, and have a retro box decode & render that instead? The server program would also be capable of dithering inline images down to monochrome for early Macs or perhaps 8-16 colors for the 8-bit micros, DOS EGA&VGA and maybe 32 colors for the Amiga 1000.
That would be better suited to the hardware limitations (including serial line transmission speeds) than trying to stuff high color images down a serial line to older systems.

SymZilla is kinda what I had in mind for systems at the lower end ( http://www.symbos.de/appinfo.htm?00007 )