[wharfrat73]

You can tell shns are seekable by running shntool info:
file name: Track 01.shn
handled by: shn format module
length: 5:22.17
WAVE format: 0x0001 (Microsoft PCM)
channels: 2
bits/sample: 16
samples/sec: 44100
average bytes/sec: 176400
rate (calculated): 176400
block align: 4
header size: 44 bytes
data size: 56840784 bytes
chunk size: 56840820 bytes
total size (chunk size + 8): 56840828 bytes
actual file size: 29095025
file is compressed: yes
compression ratio: 0.5119
CD-quality properties:
CD quality: yes
cut on sector boundary: yes
sector misalignment: 0 bytes
long enough to be burned: yes
WAVE properties:
non-canonical header: no
extra RIFF chunks: no
Possible problems:
file contains ID3v2 tag: no
data chunk block-aligned: yes
inconsistent header: no
file probably truncated: unknown
junk appended to file: unknown
odd data size has pad byte: n/a
Extra shn-specific info:
seekable: no

Granted it's a lot of reading for that one piece of information. If you set up a batch file:
@echo off
%~d1 & cd %*
for %%T in (shn flac wav ape) do if exist *.%%T shntool info *.%%T>>shntool.info.txt
pause

it's not that hard to do.

The best (by far) player I know of is foobar2000. It supports shn and flac. It generates seek tables on the fly for non seeking shns. Grab the "Special" version here.