What I did to increase volcanism was to set volcanism minimum to 20 and the variance to 1600. From my experience the variance is the key factor in getting pipes in good areas (like next to flux or randomly along a non volcanic area) as it allows the level of volcanism to vary more between areas. I get tons and tons of pipes all over the place with my world settings.
If you're curious, heres the world params I gen my worlds with:
[WORLD_GEN]
[TITLE:Balanced]
[DIM:129:129]
[END_YEAR:100]
[BEAST_END_YEAR:5000:-1]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:401:401]
[RAINFALL:0:100:1600:1600]
[TEMPERATURE:45:55:200:200]
[DRAINAGE:0:100:400:400]
[VOLCANISM:20:100:1600:1600]
[SAVAGERY:0:100:200:200]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[GOOD_SQ_COUNTS:25:251:503]
[EVIL_SQ_COUNTS:25:251:503]
[PEAK_NUMBER_MIN:0]
[OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:100]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:1040:1:1]
[REGION_COUNTS:MOUNTAINS:1040:3:1]
[REGION_COUNTS:OCEAN:0:1:1]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:1040:1:1]
[REGION_COUNTS:HILLS:1040:1:1]
[EROSION_CYCLE_COUNT:25]
[RIVER_MINS:50:50]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:100]
[MOUNTAIN_CAVE_MIN:50]
[NON_MOUNTAIN_CAVE_MIN:100]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:40]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
Using these settings I get pretty balanced worlds with lots of good starting sites. I've used it to search for small 3x3 or 4x4 sites with flux, cave river, magma pipe, and sand. Every single world I've generated has had at least one area with all of the above, and some of them have multiple locations (if you initiate the site finder from different parts of the map, it finds different sites - sometimes it can even miss a good site and show none available at one spot and find one from another spot, searching from all 4 corners can usually dig up something if you're not finding it the first time).
[VOLCANISM:20:100:1600:1600] seems to help gen tons of magma pipes, and it might be coincidence but after I put [RAINFALL:0:100:1600:1600] I noticed a lot more flux and sand next to volcanic areas.
Also [TEMPERATURE:45:55:200:200] sets the world to a stable temperature (ranging from warm to cold, no extremes) and makes more of the world liveable.
I tinkered with most of the settings (if you look I'm sure you'll spot lots of little changes) but I think those three are the ones that have had the most effect.