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I hackily solved this on the source side. A side effect of my solution is that it shows extra damage numbers (but the multihit total is still correct, and the extra damage numbers are visual only). My solution:
In the "display damage" switch block in skill_attack:
case CG_ARROWVULCAN: case AS_SONICBLOW: { dmg.dmotion = clif_skill_damage(dsrc, bl, tick, dmg.amotion, dmg.dmotion, damage, dmg.div_, skill_id, flag & SD_LEVEL ? -1 : skill_lv, dmg_type, dmg.crit); struct unit_data* ud = unit_bl2ud(dsrc); if (ud) { ud->dmg = dmg; ud->sb_animation = 0; ud->sb_target = bl->id; ud->sb_timer = add_timer(gettick()+20, skill_sonicblow_animation, dsrc->id, 0); } break; } ↑ This plays the default SB animation, sets up some data, then adds a new timer function.
Here's the code for the timer_func it adds:
TIMER_FUNC(skill_sonicblow_animation){ struct block_list *target, *src; struct unit_data *ud; struct status_change *sc = NULL; int flag = 0; src = map_id2bl(id); if(src == NULL) { return 0; } ud = unit_bl2ud(src); if(ud == NULL) { return 0; } target = map_id2bl(ud->sb_target); if (!target || ud->sb_animation >= ud->dmg.div_ || ud->sb_timer == INVALID_TIMER) { ud->sb_animation = 0; ud->sb_timer = INVALID_TIMER; return 0; } int div_ = ud->dmg.div_; if (div_ < 1) { div_ = 1; } t_tick canmove = tick_diff(ud->canmove_tick, tick); canmove /= (div_-ud->sb_animation); if (canmove > 175) { canmove = 175; } clif_damage(src, target, tick, 125, ud->dmg.dmotion/div_, ud->dmg.damage/div_, div_, DMG_NORMAL, ud->dmg.damage2/div_, ud->dmg.isspdamage); ud->sb_animation++; ud->sb_timer = add_timer(tick+canmove, skill_sonicblow_animation, id, data); return 1; } ↑ This just sets up some damage then calls clif_damage, and sets up another timer func to keep it going until sb_animation reaches skill's div count. To be clear this will work for any multiattack that you want to have spam auto attacks, not just SB. Sorry for the naming scheme.
The extra damage numbers are customizable by changing what you pass to the clif_damage call. You can set it to all 1s, or all misses, or anything else. Unfortunately I never found a way to implement this without the extra numbers—I'm not sure how to force the client to arbitrarily play an auto attack animation with no corresponding damage numbers. If someone knows, I'd be interested.
and in the unit_data struct:
int sb_animation, sb_target, sb_timer; // hacky sb animation fix ↑ just some state data, ud seemed like the best place to put it so that it works on both mobs and players.
This produces a sonic blow like in the attached file. also viewable here: https://mikomiko.org/files/Screencast_20240512_005716.webm
It's not perfect but I'm happy enough with it for now.

Screencast_20240512_005716.webm