You should be able to find a modified wpa_supplicant that enables WiFi Ad-Hoc network connections on your EVO over at xda. That said, I'd recommend that you verify the specific version of wpa_supplicant on your EVO first. Then you can locate and test the correct modified version to see if it works. The terminal command is "wpa_supplicant -v" without the quotes. With Desire HD or Z builds on the HD2, I had the v0.6.10 version, and needed to download and test a few before finally finding one that worked as intended.
To install it on the HD2, I just make sure wifi is turned off, and use Root Explorer to copy the original wpa_supplicant in /system/bin to somewhere safe on storage card. Then copy the modified version to /system/bin, fast reboot, and test away. In your case, I'm guessing you'll need to find one signed and update it through recovery.
If it helps at all, here's a link for the one that works on the latest Desire HD and Z builds:
[MISC] Enable Adhoc Networks - xda-developers
Best to all,
R
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*TMOUS HD2 with Miri & DINIK v19 ROM as well as multiple Android SD builds on a SanDisk 32GB MicroSDHC & Radio_leo_2.15.50.14
*2nd TMOUS HD2 for testing NAND DHD, Z, GB, & WP builds.
*Sprint TP2 with a solid 6.5.x ROM by Vin255764 & Radio 2.32.00WU