Can I create a Picker of Process Models?

You can try custom picker. Following is SAIL recepie code for custom picker that I modified to update for process models (Appian version 16.3):

=load(
local!pickedprocessModels: {},
local!processModelLabels: index(getprocessmodelsbyfolder(32,a!pagingInfo(startIndex:1,batchSize:4 )),"name"),
local!processModelAbbreviations: index(getprocessmodelsbyfolder(32,a!pagingInfo(startIndex:1,batchSize:4 )),"id"),
a!formLayout(
firstColumnContents: {
a!pickerFieldCustom(
label: "Process Models",

maxSelections: 3,
suggestFunction: rule!test_filterRule(filter: _, labels: local!processModelLabels, identifiers: local!processModelAbbreviations),
selectedLabels: if(
count(local!pickedprocessModels) = 0,
null,
apply(
rule!test_FilterDisplayRule(identifier: _, labels: local!processModelLabels, identifiers: local!processModelAbbreviations),
local!pickedprocessModels
)
),
value: local!pickedprocessModels,
saveInto: local!pickedprocessModels
),
a!textField(
label: "Selected Data",
value: toprocessmodel(local!pickedprocessModels),
readOnly: true
)
}
)
)

Note: getprocessmodelsbyfolder is a custom function which returns all the details of process models in a folder. You can use constants of type process model and text with uuids instead but then you'll have dependency on the models.

You can try custom picker. Following is SAIL recepie code for custom picker that I modified to update for process models (Appian version 16.3):

=load(
local!pickedprocessModels: {},
local!processModelLabels: index(getprocessmodelsbyfolder(32,a!pagingInfo(startIndex:1,batchSize:4 )),"name"),
local!processModelAbbreviations: index(getprocessmodelsbyfolder(32,a!pagingInfo(startIndex:1,batchSize:4 )),"id"),
a!formLayout(
firstColumnContents: {
a!pickerFieldCustom(
label: "Process Models",

maxSelections: 3,
suggestFunction: rule!test_filterRule(filter: _, labels: local!processModelLabels, identifiers: local!processModelAbbreviations),
selectedLabels: if(
count(local!pickedprocessModels) = 0,
null,
apply(
rule!test_FilterDisplayRule(identifier: _, labels: local!processModelLabels, identifiers: local!processModelAbbreviations),
local!pickedprocessModels
)
),
value: local!pickedprocessModels,
saveInto: local!pickedprocessModels
),
a!textField(
label: "Selected Data",
value: toprocessmodel(local!pickedprocessModels),
readOnly: true
)
}
)
)

Note: getprocessmodelsbyfolder is a custom function which returns all the details of process models in a folder. You can use constants of type process model and text with uuids instead but then you'll have dependency on the models.

Thanks, that's a good potential solution. Is getprocessmodelsbyfolder available as a plugin on Forum?

Unfortunately it's an internal plugin and I don't have access to the plugin jar. But you can develop the plugin by using following plugin source code as reference :
forum.appian.com/.../summary

Following is the link of the API interface where you'll be able to find the methods to implement this plugin

docs.appian.com/.../ProcessDesignService.html