QMP doesn't support them in its UI, but there are individual sip and puff preferences that can be set to override the default sip_puff_... preferences on the QMP misc tab:
const keyword_t preference_keywords[NUMBER_OF_PREFS] = {
{"sip_puff_threshold_soft", SIP_PUFF_THRESHOLD_LOW},
{"sip_puff_threshold", SIP_PUFF_THRESHOLD_HIGH},
{"sip_puff_maximum", SIP_PUFF_MAXIMUM},
{"sip_puff_delay_soft", SIP_PUFF_DELAY},
{"sip_puff_delay_hard", SIP_PUFF_DELAY_HARD},
{"sip_threshold_soft", SIP_THRESHOLD_LOW},
{"sip_threshold", SIP_THRESHOLD_HIGH},
{"sip_maximum", SIP_MAXIMUM},
{"puff_threshold_soft", PUFF_THRESHOLD_LOW},
{"puff_threshold", PUFF_THRESHOLD_HIGH},
{"puff_maximum", PUFF_MAXIMUM},