So I was trying some old games and I found Line Wars 2 (previously shareware, now freeware).

It supports a (small) variety of graphics cards, MCGA (Standard VGA), VGA (320x480, mode X), VESA (640x480), S3 (800x600) and ATI Mach 8 (800x600). My Dosbox.conf file include the line machine=svga_s3 so it should emulate a S3.

The results:
- The game autodetects the S3 card. It shows it as an "S3 Accelerated SVGA", with unknown chipset and 2048Kb RAM.
- MCGA, VGA and VESA modes works fine.
- S3 mode shows this message: S3DRV ERROR: VESA Enhanced 800x600 mode with 256 colors not available!
- The ATI mode shows that the card is not detected (obviously, AFAIK Dosbox doesn't emulate it).

The S3 emulation is working (it gets detected), and using whatvga (from VGADOC) shows both VESA and S3 (listed as a S3 Trio64 764). That's normal (my old Trident card was listed as both a VESA and a Trident card), and it lists a 800x600x256c mode as 0x103 on both VESA and S3 modes list.

Other tests:
- UniVBE 4.1 (included with download) installs as VESA 2.0, the game doesn't work.
- UniVBE 6.53 installs VESA 2.0 and 3.0 extensions, the game doesn't work.
- PCem v17 (using a 486 with S3 Trio 32 card) does work.
- My old laptop (S3 Savage MX) tries to start the game, then hangs. Not a big surprise, every other game with S3 support that I've tested didn't work.